-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. Neither has
the pcredemo program. There are separate text files for the pcregrep and
pcretest commands.
-----------------------------------------------------------------------------


PCRE(3)                                                                PCRE(3)


NAME
       PCRE - Perl-compatible regular expressions


INTRODUCTION

       The  PCRE  library is a set of functions that implement regular expres-
       sion pattern matching using the same syntax and semantics as Perl, with
       just  a few differences. Some features that appeared in Python and PCRE
       before they appeared in Perl are also available using the  Python  syn-
       tax,  there  is  some  support for one or two .NET and Oniguruma syntax
       items, and there is an option for requesting some  minor  changes  that
       give better JavaScript compatibility.

       Starting with release 8.30, it is possible to compile two separate PCRE
       libraries:  the  original,  which  supports  8-bit  character   strings
       (including  UTF-8  strings),  and a second library that supports 16-bit
       character strings (including UTF-16 strings). The build process  allows
       either  one  or both to be built. The majority of the work to make this
       possible was done by Zoltan Herczeg.

       The two libraries contain identical sets of functions, except that  the
       names  in  the  16-bit  library start with pcre16_ instead of pcre_. To
       avoid over-complication and reduce the documentation maintenance  load,
       most of the documentation describes the 8-bit library, with the differ-
       ences for the 16-bit library described separately in the  pcre16  page.
       References  to  functions or structures of the form pcre[16]_xxx should
       be  read  as  meaning  "pcre_xxx  when  using  the  8-bit  library  and
       pcre16_xxx when using the 16-bit library".

       The  current implementation of PCRE corresponds approximately with Perl
       5.12, including support for UTF-8/16 encoded strings and  Unicode  gen-
       eral  category properties. However, UTF-8/16 and Unicode support has to
       be explicitly enabled; it is not the default. The Unicode tables corre-
       spond to Unicode release 6.0.0.

       In  addition to the Perl-compatible matching function, PCRE contains an
       alternative function that matches the same compiled patterns in a  dif-
       ferent way. In certain circumstances, the alternative function has some
       advantages.  For a discussion of the two matching algorithms,  see  the
       pcrematching page.

       PCRE  is  written  in C and released as a C library. A number of people
       have written wrappers and interfaces of various kinds.  In  particular,
       Google  Inc.   have  provided a comprehensive C++ wrapper for the 8-bit
       library. This is now included as part of  the  PCRE  distribution.  The
       pcrecpp  page  has  details of this interface. Other people's contribu-
       tions can be found in the Contrib directory at the  primary  FTP  site,
       which is:

       ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre

       Details  of  exactly which Perl regular expression features are and are
       not supported by PCRE are given in separate documents. See the pcrepat-
       tern  and pcrecompat pages. There is a syntax summary in the pcresyntax
       page.

       Some features of PCRE can be included, excluded, or  changed  when  the
       library  is  built.  The pcre_config() function makes it possible for a
       client to discover which features are  available.  The  features  them-
       selves  are described in the pcrebuild page. Documentation about build-
       ing PCRE for various operating systems can be found in the  README  and
       NON-UNIX-USE files in the source distribution.

       The  libraries contains a number of undocumented internal functions and
       data tables that are used by more than one  of  the  exported  external
       functions,  but  which  are  not  intended for use by external callers.
       Their names all begin with "_pcre_" or "_pcre16_", which hopefully will
       not  provoke  any name clashes. In some environments, it is possible to
       control which external symbols are exported when a  shared  library  is
       built, and in these cases the undocumented symbols are not exported.


USER DOCUMENTATION

       The  user  documentation  for PCRE comprises a number of different sec-
       tions. In the "man" format, each of these is a separate "man page".  In
       the  HTML  format, each is a separate page, linked from the index page.
       In the plain text format, all the sections, except  the  pcredemo  sec-
       tion, are concatenated, for ease of searching. The sections are as fol-
       lows:

         pcre              this document
         pcre16            details of the 16-bit library
         pcre-config       show PCRE installation configuration information
         pcreapi           details of PCRE's native C API
         pcrebuild         options for building PCRE
         pcrecallout       details of the callout feature
         pcrecompat        discussion of Perl compatibility
         pcrecpp           details of the C++ wrapper for the 8-bit library
         pcredemo          a demonstration C program that uses PCRE
         pcregrep          description of the pcregrep command (8-bit only)
         pcrejit           discussion of the just-in-time optimization support
         pcrelimits        details of size and other limits
         pcrematching      discussion of the two matching algorithms
         pcrepartial       details of the partial matching facility
         pcrepattern       syntax and semantics of supported
                             regular expressions
         pcreperform       discussion of performance issues
         pcreposix         the POSIX-compatible C API for the 8-bit library
         pcreprecompile    details of saving and re-using precompiled patterns
         pcresample        discussion of the pcredemo program
         pcrestack         discussion of stack usage
         pcresyntax        quick syntax reference
         pcretest          description of the pcretest testing command
         pcreunicode       discussion of Unicode and UTF-8/16 support

       In addition, in the "man" and HTML formats, there is a short  page  for
       each 8-bit C library function, listing its arguments and results.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

       Putting  an actual email address here seems to have been a spam magnet,
       so I've taken it away. If you want to email me, use  my  two  initials,
       followed by the two digits 10, at the domain cam.ac.uk.


REVISION

       Last updated: 10 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRE(3)                                                                PCRE(3)


NAME
       PCRE - Perl-compatible regular expressions

       #include <pcre.h>


PCRE 16-BIT API BASIC FUNCTIONS

       pcre16 *pcre16_compile(PCRE_SPTR16 pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre16 *pcre16_compile2(PCRE_SPTR16 pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre16_extra *pcre16_study(const pcre16 *code, int options,
            const char **errptr);

       void pcre16_free_study(pcre16_extra *extra);

       int pcre16_exec(const pcre16 *code, const pcre16_extra *extra,
            PCRE_SPTR16 subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       int pcre16_dfa_exec(const pcre16 *code, const pcre16_extra *extra,
            PCRE_SPTR16 subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);


PCRE 16-BIT API STRING EXTRACTION FUNCTIONS

       int pcre16_copy_named_substring(const pcre16 *code,
            PCRE_SPTR16 subject, int *ovector,
            int stringcount, PCRE_SPTR16 stringname,
            PCRE_UCHAR16 *buffer, int buffersize);

       int pcre16_copy_substring(PCRE_SPTR16 subject, int *ovector,
            int stringcount, int stringnumber, PCRE_UCHAR16 *buffer,
            int buffersize);

       int pcre16_get_named_substring(const pcre16 *code,
            PCRE_SPTR16 subject, int *ovector,
            int stringcount, PCRE_SPTR16 stringname,
            PCRE_SPTR16 *stringptr);

       int pcre16_get_stringnumber(const pcre16 *code,
            PCRE_SPTR16 name);

       int pcre16_get_stringtable_entries(const pcre16 *code,
            PCRE_SPTR16 name, PCRE_UCHAR16 **first, PCRE_UCHAR16 **last);

       int pcre16_get_substring(PCRE_SPTR16 subject, int *ovector,
            int stringcount, int stringnumber,
            PCRE_SPTR16 *stringptr);

       int pcre16_get_substring_list(PCRE_SPTR16 subject,
            int *ovector, int stringcount, PCRE_SPTR16 **listptr);

       void pcre16_free_substring(PCRE_SPTR16 stringptr);

       void pcre16_free_substring_list(PCRE_SPTR16 *stringptr);


PCRE 16-BIT API AUXILIARY FUNCTIONS

       pcre16_jit_stack *pcre16_jit_stack_alloc(int startsize, int maxsize);

       void pcre16_jit_stack_free(pcre16_jit_stack *stack);

       void pcre16_assign_jit_stack(pcre16_extra *extra,
            pcre16_jit_callback callback, void *data);

       const unsigned char *pcre16_maketables(void);

       int pcre16_fullinfo(const pcre16 *code, const pcre16_extra *extra,
            int what, void *where);

       int pcre16_refcount(pcre16 *code, int adjust);

       int pcre16_config(int what, void *where);

       const char *pcre16_version(void);

       int pcre16_pattern_to_host_byte_order(pcre16 *code,
            pcre16_extra *extra, const unsigned char *tables);


PCRE 16-BIT API INDIRECTED FUNCTIONS

       void *(*pcre16_malloc)(size_t);

       void (*pcre16_free)(void *);

       void *(*pcre16_stack_malloc)(size_t);

       void (*pcre16_stack_free)(void *);

       int (*pcre16_callout)(pcre16_callout_block *);


PCRE 16-BIT API 16-BIT-ONLY FUNCTION

       int pcre16_utf16_to_host_byte_order(PCRE_UCHAR16 *output,
            PCRE_SPTR16 input, int length, int *byte_order,
            int keep_boms);


THE PCRE 16-BIT LIBRARY

       Starting  with  release  8.30, it is possible to compile a PCRE library
       that supports 16-bit character strings, including  UTF-16  strings,  as
       well  as  or instead of the original 8-bit library. The majority of the
       work to make  this  possible  was  done  by  Zoltan  Herczeg.  The  two
       libraries contain identical sets of functions, used in exactly the same
       way. Only the names of the functions and the data types of their  argu-
       ments  and results are different. To avoid over-complication and reduce
       the documentation maintenance load,  most  of  the  PCRE  documentation
       describes  the  8-bit  library,  with only occasional references to the
       16-bit library. This page describes what is different when you use  the
       16-bit library.

       WARNING:  A  single  application can be linked with both libraries, but
       you must take care when processing any particular pattern to use  func-
       tions  from  just one library. For example, if you want to study a pat-
       tern that was compiled with  pcre16_compile(),  you  must  do  so  with
       pcre16_study(), not pcre_study(), and you must free the study data with
       pcre16_free_study().


THE HEADER FILE

       There is only one header file, pcre.h. It contains prototypes  for  all
       the  functions  in  both  libraries,  as  well as definitions of flags,
       structures, error codes, etc.


THE LIBRARY NAME

       In Unix-like systems, the 16-bit library is called libpcre16,  and  can
       normally  be  accesss  by adding -lpcre16 to the command for linking an
       application that uses PCRE.


STRING TYPES

       In the 8-bit library, strings are passed to PCRE library  functions  as
       vectors  of  bytes  with  the  C  type "char *". In the 16-bit library,
       strings are passed as vectors of unsigned 16-bit quantities. The  macro
       PCRE_UCHAR16  specifies  an  appropriate  data type, and PCRE_SPTR16 is
       defined as "const PCRE_UCHAR16 *". In very  many  environments,  "short
       int" is a 16-bit data type. When PCRE is built, it defines PCRE_UCHAR16
       as "short int", but checks that it really is a 16-bit data type. If  it
       is not, the build fails with an error message telling the maintainer to
       modify the definition appropriately.


STRUCTURE TYPES

       The types of the opaque structures that are used  for  compiled  16-bit
       patterns  and  JIT stacks are pcre16 and pcre16_jit_stack respectively.
       The  type  of  the  user-accessible  structure  that  is  returned   by
       pcre16_study()  is  pcre16_extra, and the type of the structure that is
       used for passing data to a callout  function  is  pcre16_callout_block.
       These structures contain the same fields, with the same names, as their
       8-bit counterparts. The only difference is that pointers  to  character
       strings are 16-bit instead of 8-bit types.


16-BIT FUNCTIONS

       For  every function in the 8-bit library there is a corresponding func-
       tion in the 16-bit library with a name that starts with pcre16_ instead
       of  pcre_.  The  prototypes are listed above. In addition, there is one
       extra function, pcre16_utf16_to_host_byte_order(). This  is  a  utility
       function  that converts a UTF-16 character string to host byte order if
       necessary. The other 16-bit  functions  expect  the  strings  they  are
       passed to be in host byte order.

       The input and output arguments of pcre16_utf16_to_host_byte_order() may
       point to the same address, that is, conversion in place  is  supported.
       The output buffer must be at least as long as the input.

       The  length  argument  specifies the number of 16-bit data units in the
       input string; a negative value specifies a zero-terminated string.

       If byte_order is NULL, it is assumed that the string starts off in host
       byte  order. This may be changed by byte-order marks (BOMs) anywhere in
       the string (commonly as the first character).

       If byte_order is not NULL, a non-zero value of the integer to which  it
       points  means  that  the input starts off in host byte order, otherwise
       the opposite order is assumed. Again, BOMs in  the  string  can  change
       this. The final byte order is passed back at the end of processing.

       If  keep_boms  is  not  zero,  byte-order  mark characters (0xfeff) are
       copied into the output string. Otherwise they are discarded.

       The result of the function is the number of 16-bit  units  placed  into
       the  output  buffer,  including  the  zero terminator if the string was
       zero-terminated.


SUBJECT STRING OFFSETS

       The offsets within subject strings that are returned  by  the  matching
       functions are in 16-bit units rather than bytes.


NAMED SUBPATTERNS

       The  name-to-number translation table that is maintained for named sub-
       patterns uses 16-bit characters.  The  pcre16_get_stringtable_entries()
       function returns the length of each entry in the table as the number of
       16-bit data units.


OPTION NAMES

       There   are   two   new   general   option   names,   PCRE_UTF16    and
       PCRE_NO_UTF16_CHECK,     which     correspond    to    PCRE_UTF8    and
       PCRE_NO_UTF8_CHECK in the 8-bit library. In  fact,  these  new  options
       define  the  same bits in the options word. There is a discussion about
       the validity of UTF-16 strings in the pcreunicode page.

       For the pcre16_config() function there is an  option  PCRE_CONFIG_UTF16
       that  returns  1  if UTF-16 support is configured, otherwise 0. If this
       option is given to pcre_config(), or if the PCRE_CONFIG_UTF8 option  is
       given to pcre16_config(), the result is the PCRE_ERROR_BADOPTION error.


CHARACTER CODES

       In  16-bit  mode,  when  PCRE_UTF16  is  not  set, character values are
       treated in the same way as in 8-bit, non UTF-8 mode, except, of course,
       that  they  can  range from 0 to 0xffff instead of 0 to 0xff. Character
       types for characters less than 0xff can therefore be influenced by  the
       locale  in  the  same way as before.  Characters greater than 0xff have
       only one case, and no "type" (such as letter or digit).

       In UTF-16 mode, the character code  is  Unicode,  in  the  range  0  to
       0x10ffff,  with  the  exception of values in the range 0xd800 to 0xdfff
       because those are "surrogate" values that are used in pairs  to  encode
       values greater than 0xffff.

       A  UTF-16 string can indicate its endianness by special code knows as a
       byte-order mark (BOM). The PCRE functions do not handle this, expecting
       strings   to   be  in  host  byte  order.  A  utility  function  called
       pcre16_utf16_to_host_byte_order() is provided to help  with  this  (see
       above).


ERROR NAMES

       The  errors PCRE_ERROR_BADUTF16_OFFSET and PCRE_ERROR_SHORTUTF16 corre-
       spond to their 8-bit  counterparts.  The  error  PCRE_ERROR_BADMODE  is
       given  when  a  compiled pattern is passed to a function that processes
       patterns in the other mode, for example, if  a  pattern  compiled  with
       pcre_compile() is passed to pcre16_exec().

       There  are  new  error  codes whose names begin with PCRE_UTF16_ERR for
       invalid UTF-16 strings, corresponding to the  PCRE_UTF8_ERR  codes  for
       UTF-8  strings that are described in the section entitled "Reason codes
       for invalid UTF-8 strings" in the main pcreapi page. The UTF-16  errors
       are:

         PCRE_UTF16_ERR1  Missing low surrogate at end of string
         PCRE_UTF16_ERR2  Invalid low surrogate follows high surrogate
         PCRE_UTF16_ERR3  Isolated low surrogate
         PCRE_UTF16_ERR4  Invalid character 0xfffe


ERROR TEXTS

       If  there is an error while compiling a pattern, the error text that is
       passed back by pcre16_compile() or pcre16_compile2() is still an  8-bit
       character string, zero-terminated.


CALLOUTS

       The  subject  and  mark fields in the callout block that is passed to a
       callout function point to 16-bit vectors.


TESTING

       The pcretest program continues to operate with 8-bit input  and  output
       files,  but it can be used for testing the 16-bit library. If it is run
       with the command line option -16, patterns and subject strings are con-
       verted from 8-bit to 16-bit before being passed to PCRE, and the 16-bit
       library functions are used instead of the 8-bit ones.  Returned  16-bit
       strings are converted to 8-bit for output. If the 8-bit library was not
       compiled, pcretest defaults to 16-bit and the -16 option is ignored.

       When PCRE is being built, the RunTest script that is  called  by  "make
       check"  uses  the pcretest -C option to discover which of the 8-bit and
       16-bit libraries has been built, and runs the tests appropriately.


NOT SUPPORTED IN 16-BIT MODE

       Not all the features of the 8-bit library are available with the 16-bit
       library.  The  C++  and  POSIX wrapper functions support only the 8-bit
       library, and the pcregrep program is at present 8-bit only.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 14 April 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREBUILD(3)                                                      PCREBUILD(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE BUILD-TIME OPTIONS

       This  document  describes  the  optional  features  of PCRE that can be
       selected when the library is compiled. It assumes use of the  configure
       script,  where the optional features are selected or deselected by pro-
       viding options to configure before running the make  command.  However,
       the  same  options  can be selected in both Unix-like and non-Unix-like
       environments using the GUI facility of cmake-gui if you are using CMake
       instead of configure to build PCRE.

       There  is  a  lot more information about building PCRE in non-Unix-like
       environments in the file called NON_UNIX_USE, which is part of the PCRE
       distribution.  You  should consult this file as well as the README file
       if you are building in a non-Unix-like environment.

       The complete list of options for configure (which includes the standard
       ones  such  as  the  selection  of  the  installation directory) can be
       obtained by running

         ./configure --help

       The following sections include  descriptions  of  options  whose  names
       begin with --enable or --disable. These settings specify changes to the
       defaults for the configure command. Because of the way  that  configure
       works,  --enable  and --disable always come in pairs, so the complemen-
       tary option always exists as well, but as it specifies the default,  it
       is not described.


BUILDING 8-BIT and 16-BIT LIBRARIES

       By  default,  a  library  called libpcre is built, containing functions
       that take string arguments contained in vectors  of  bytes,  either  as
       single-byte  characters,  or interpreted as UTF-8 strings. You can also
       build a separate library, called libpcre16, in which strings  are  con-
       tained  in  vectors of 16-bit data units and interpreted either as sin-
       gle-unit characters or UTF-16 strings, by adding

         --enable-pcre16

       to the configure command. If you do not want the 8-bit library, add

         --disable-pcre8

       as well. At least one of the two libraries must be built. Note that the
       C++  and  POSIX wrappers are for the 8-bit library only, and that pcre-
       grep is an 8-bit program. None of these are built if  you  select  only
       the 16-bit library.


BUILDING SHARED AND STATIC LIBRARIES

       The  PCRE building process uses libtool to build both shared and static
       Unix libraries by default. You can suppress one of these by adding  one
       of

         --disable-shared
         --disable-static

       to the configure command, as required.


C++ SUPPORT

       By  default,  if the 8-bit library is being built, the configure script
       will search for a C++ compiler and C++ header files. If it finds  them,
       it  automatically  builds  the C++ wrapper library (which supports only
       8-bit strings). You can disable this by adding

         --disable-cpp

       to the configure command.


UTF-8 and UTF-16 SUPPORT

       To build PCRE with support for UTF Unicode character strings, add

         --enable-utf

       to the configure command.  This  setting  applies  to  both  libraries,
       adding support for UTF-8 to the 8-bit library and support for UTF-16 to
       the 16-bit library. There are no separate options  for  enabling  UTF-8
       and  UTF-16  independently because that would allow ridiculous settings
       such as  requesting  UTF-16  support  while  building  only  the  8-bit
       library.  It  is not possible to build one library with UTF support and
       the other without in the same configuration. (For backwards compatibil-
       ity, --enable-utf8 is a synonym of --enable-utf.)

       Of  itself,  this  setting does not make PCRE treat strings as UTF-8 or
       UTF-16. As well as compiling PCRE with this option, you also have  have
       to set the PCRE_UTF8 or PCRE_UTF16 option when you call one of the pat-
       tern compiling functions.

       If you set --enable-utf when compiling in an EBCDIC  environment,  PCRE
       expects  its  input  to be either ASCII or UTF-8 (depending on the run-
       time option). It is not possible to support both EBCDIC and UTF-8 codes
       in  the  same  version  of  the library. Consequently, --enable-utf and
       --enable-ebcdic are mutually exclusive.


UNICODE CHARACTER PROPERTY SUPPORT

       UTF support allows the libraries to process character codepoints up  to
       0x10ffff  in the strings that they handle. On its own, however, it does
       not provide any facilities for accessing the properties of such charac-
       ters. If you want to be able to use the pattern escapes \P, \p, and \X,
       which refer to Unicode character properties, you must add

         --enable-unicode-properties

       to the configure command. This implies UTF support, even  if  you  have
       not explicitly requested it.

       Including  Unicode  property  support  adds around 30K of tables to the
       PCRE library. Only the general category properties such as  Lu  and  Nd
       are supported. Details are given in the pcrepattern documentation.


JUST-IN-TIME COMPILER SUPPORT

       Just-in-time compiler support is included in the build by specifying

         --enable-jit

       This  support  is available only for certain hardware architectures. If
       this option is set for an  unsupported  architecture,  a  compile  time
       error  occurs.   See  the pcrejit documentation for a discussion of JIT
       usage. When JIT support is enabled, pcregrep automatically makes use of
       it, unless you add

         --disable-pcregrep-jit

       to the "configure" command.


CODE VALUE OF NEWLINE

       By  default,  PCRE interprets the linefeed (LF) character as indicating
       the end of a line. This is the normal newline  character  on  Unix-like
       systems.  You  can compile PCRE to use carriage return (CR) instead, by
       adding

         --enable-newline-is-cr

       to the  configure  command.  There  is  also  a  --enable-newline-is-lf
       option, which explicitly specifies linefeed as the newline character.

       Alternatively, you can specify that line endings are to be indicated by
       the two character sequence CRLF. If you want this, add

         --enable-newline-is-crlf

       to the configure command. There is a fourth option, specified by

         --enable-newline-is-anycrlf

       which causes PCRE to recognize any of the three sequences  CR,  LF,  or
       CRLF as indicating a line ending. Finally, a fifth option, specified by

         --enable-newline-is-any

       causes PCRE to recognize any Unicode newline sequence.

       Whatever  line  ending convention is selected when PCRE is built can be
       overridden when the library functions are called. At build time  it  is
       conventional to use the standard for your operating system.


WHAT \R MATCHES

       By  default,  the  sequence \R in a pattern matches any Unicode newline
       sequence, whatever has been selected as the line  ending  sequence.  If
       you specify

         --enable-bsr-anycrlf

       the  default  is changed so that \R matches only CR, LF, or CRLF. What-
       ever is selected when PCRE is built can be overridden when the  library
       functions are called.


POSIX MALLOC USAGE

       When  the  8-bit library is called through the POSIX interface (see the
       pcreposix documentation), additional working storage  is  required  for
       holding  the  pointers  to  capturing substrings, because PCRE requires
       three integers per substring, whereas the POSIX interface provides only
       two.  If  the number of expected substrings is small, the wrapper func-
       tion uses space on the stack, because this is faster  than  using  mal-
       loc()  for each call. The default threshold above which the stack is no
       longer used is 10; it can be changed by adding a setting such as

         --with-posix-malloc-threshold=20

       to the configure command.


HANDLING VERY LARGE PATTERNS

       Within a compiled pattern, offset values are used  to  point  from  one
       part  to another (for example, from an opening parenthesis to an alter-
       nation metacharacter). By default, two-byte values are used  for  these
       offsets,  leading  to  a  maximum size for a compiled pattern of around
       64K. This is sufficient to handle all but the most  gigantic  patterns.
       Nevertheless,  some  people do want to process truly enormous patterns,
       so it is possible to compile PCRE to use three-byte or  four-byte  off-
       sets by adding a setting such as

         --with-link-size=3

       to  the  configure command. The value given must be 2, 3, or 4. For the
       16-bit library, a value of 3 is rounded up to 4. Using  longer  offsets
       slows down the operation of PCRE because it has to load additional data
       when handling them.


AVOIDING EXCESSIVE STACK USAGE

       When matching with the pcre_exec() function, PCRE implements backtrack-
       ing  by  making recursive calls to an internal function called match().
       In environments where the size of the stack is limited,  this  can  se-
       verely  limit  PCRE's operation. (The Unix environment does not usually
       suffer from this problem, but it may sometimes be necessary to increase
       the  maximum  stack size.  There is a discussion in the pcrestack docu-
       mentation.) An alternative approach to recursion that uses memory  from
       the  heap  to remember data, instead of using recursive function calls,
       has been implemented to work round the problem of limited  stack  size.
       If you want to build a version of PCRE that works this way, add

         --disable-stack-for-recursion

       to  the  configure  command. With this configuration, PCRE will use the
       pcre_stack_malloc and pcre_stack_free variables to call memory  manage-
       ment  functions. By default these point to malloc() and free(), but you
       can replace the pointers so that your own functions are used instead.

       Separate functions are  provided  rather  than  using  pcre_malloc  and
       pcre_free  because  the  usage  is  very  predictable:  the block sizes
       requested are always the same, and  the  blocks  are  always  freed  in
       reverse  order.  A calling program might be able to implement optimized
       functions that perform better  than  malloc()  and  free().  PCRE  runs
       noticeably more slowly when built in this way. This option affects only
       the pcre_exec() function; it is not relevant for pcre_dfa_exec().


LIMITING PCRE RESOURCE USAGE

       Internally, PCRE has a function called match(), which it calls  repeat-
       edly   (sometimes   recursively)  when  matching  a  pattern  with  the
       pcre_exec() function. By controlling the maximum number of  times  this
       function  may be called during a single matching operation, a limit can
       be placed on the resources used by a single call  to  pcre_exec().  The
       limit  can be changed at run time, as described in the pcreapi documen-
       tation. The default is 10 million, but this can be changed by adding  a
       setting such as

         --with-match-limit=500000

       to   the   configure  command.  This  setting  has  no  effect  on  the
       pcre_dfa_exec() matching function.

       In some environments it is desirable to limit the  depth  of  recursive
       calls of match() more strictly than the total number of calls, in order
       to restrict the maximum amount of stack (or heap,  if  --disable-stack-
       for-recursion is specified) that is used. A second limit controls this;
       it defaults to the value that  is  set  for  --with-match-limit,  which
       imposes  no  additional constraints. However, you can set a lower limit
       by adding, for example,

         --with-match-limit-recursion=10000

       to the configure command. This value can  also  be  overridden  at  run
       time.


CREATING CHARACTER TABLES AT BUILD TIME

       PCRE  uses fixed tables for processing characters whose code values are
       less than 256. By default, PCRE is built with a set of tables that  are
       distributed  in  the  file pcre_chartables.c.dist. These tables are for
       ASCII codes only. If you add

         --enable-rebuild-chartables

       to the configure command, the distributed tables are  no  longer  used.
       Instead,  a  program  called dftables is compiled and run. This outputs
       the source for new set of tables, created in the default locale of your
       C  run-time  system. (This method of replacing the tables does not work
       if you are cross compiling, because dftables is run on the local  host.
       If you need to create alternative tables when cross compiling, you will
       have to do so "by hand".)


USING EBCDIC CODE

       PCRE assumes by default that it will run in an  environment  where  the
       character  code  is  ASCII  (or Unicode, which is a superset of ASCII).
       This is the case for most computer operating systems.  PCRE  can,  how-
       ever, be compiled to run in an EBCDIC environment by adding

         --enable-ebcdic

       to the configure command. This setting implies --enable-rebuild-charta-
       bles. You should only use it if you know that  you  are  in  an  EBCDIC
       environment  (for  example,  an  IBM  mainframe  operating system). The
       --enable-ebcdic option is incompatible with --enable-utf.


PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT

       By default, pcregrep reads all files as plain text. You can build it so
       that it recognizes files whose names end in .gz or .bz2, and reads them
       with libz or libbz2, respectively, by adding one or both of

         --enable-pcregrep-libz
         --enable-pcregrep-libbz2

       to the configure command. These options naturally require that the rel-
       evant  libraries  are installed on your system. Configuration will fail
       if they are not.


PCREGREP BUFFER SIZE

       pcregrep uses an internal buffer to hold a "window" on the file  it  is
       scanning, in order to be able to output "before" and "after" lines when
       it finds a match. The size of the buffer is controlled by  a  parameter
       whose default value is 20K. The buffer itself is three times this size,
       but because of the way it is used for holding "before" lines, the long-
       est  line  that  is guaranteed to be processable is the parameter size.
       You can change the default parameter value by adding, for example,

         --with-pcregrep-bufsize=50K

       to the configure command. The caller of pcregrep can, however, override
       this value by specifying a run-time option.


PCRETEST OPTION FOR LIBREADLINE SUPPORT

       If you add

         --enable-pcretest-libreadline

       to  the  configure  command,  pcretest  is  linked with the libreadline
       library, and when its input is from a terminal, it reads it  using  the
       readline() function. This provides line-editing and history facilities.
       Note that libreadline is GPL-licensed, so if you distribute a binary of
       pcretest linked in this way, there may be licensing issues.

       Setting  this  option  causes  the -lreadline option to be added to the
       pcretest build. In many operating environments with  a  sytem-installed
       libreadline this is sufficient. However, in some environments (e.g.  if
       an unmodified distribution version of readline is in use),  some  extra
       configuration  may  be necessary. The INSTALL file for libreadline says
       this:

         "Readline uses the termcap functions, but does not link with the
         termcap or curses library itself, allowing applications which link
         with readline the to choose an appropriate library."

       If your environment has not been set up so that an appropriate  library
       is automatically included, you may need to add something like

         LIBS="-ncurses"

       immediately before the configure command.


SEE ALSO

       pcreapi(3), pcre16, pcre_config(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 07 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREMATCHING(3)                                                PCREMATCHING(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE MATCHING ALGORITHMS

       This document describes the two different algorithms that are available
       in PCRE for matching a compiled regular expression against a given sub-
       ject  string.  The  "standard"  algorithm  is  the  one provided by the
       pcre_exec() and pcre16_exec() functions. These work in the same was  as
       Perl's matching function, and provide a Perl-compatible matching opera-
       tion. The just-in-time (JIT) optimization  that  is  described  in  the
       pcrejit documentation is compatible with these functions.

       An  alternative  algorithm  is  provided  by  the  pcre_dfa_exec()  and
       pcre16_dfa_exec() functions; they operate in a different way,  and  are
       not  Perl-compatible. This alternative has advantages and disadvantages
       compared with the standard algorithm, and these are described below.

       When there is only one possible way in which a given subject string can
       match  a pattern, the two algorithms give the same answer. A difference
       arises, however, when there are multiple possibilities. For example, if
       the pattern

         ^<.*>

       is matched against the string

         <something> <something else> <something further>

       there are three possible answers. The standard algorithm finds only one
       of them, whereas the alternative algorithm finds all three.


REGULAR EXPRESSIONS AS TREES

       The set of strings that are matched by a regular expression can be rep-
       resented  as  a  tree structure. An unlimited repetition in the pattern
       makes the tree of infinite size, but it is still a tree.  Matching  the
       pattern  to a given subject string (from a given starting point) can be
       thought of as a search of the tree.  There are two  ways  to  search  a
       tree:  depth-first  and  breadth-first, and these correspond to the two
       matching algorithms provided by PCRE.


THE STANDARD MATCHING ALGORITHM

       In the terminology of Jeffrey Friedl's book "Mastering Regular  Expres-
       sions",  the  standard  algorithm  is an "NFA algorithm". It conducts a
       depth-first search of the pattern tree. That is, it  proceeds  along  a
       single path through the tree, checking that the subject matches what is
       required. When there is a mismatch, the algorithm  tries  any  alterna-
       tives  at  the  current point, and if they all fail, it backs up to the
       previous branch point in the  tree,  and  tries  the  next  alternative
       branch  at  that  level.  This often involves backing up (moving to the
       left) in the subject string as well.  The  order  in  which  repetition
       branches  are  tried  is controlled by the greedy or ungreedy nature of
       the quantifier.

       If a leaf node is reached, a matching string has  been  found,  and  at
       that  point the algorithm stops. Thus, if there is more than one possi-
       ble match, this algorithm returns the first one that it finds.  Whether
       this  is the shortest, the longest, or some intermediate length depends
       on the way the greedy and ungreedy repetition quantifiers are specified
       in the pattern.

       Because  it  ends  up  with a single path through the tree, it is rela-
       tively straightforward for this algorithm to keep  track  of  the  sub-
       strings  that  are  matched  by portions of the pattern in parentheses.
       This provides support for capturing parentheses and back references.


THE ALTERNATIVE MATCHING ALGORITHM

       This algorithm conducts a breadth-first search of  the  tree.  Starting
       from  the  first  matching  point  in the subject, it scans the subject
       string from left to right, once, character by character, and as it does
       this,  it remembers all the paths through the tree that represent valid
       matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
       though  it is not implemented as a traditional finite state machine (it
       keeps multiple states active simultaneously).

       Although the general principle of this matching algorithm  is  that  it
       scans  the subject string only once, without backtracking, there is one
       exception: when a lookaround assertion is encountered,  the  characters
       following  or  preceding  the  current  point  have to be independently
       inspected.

       The scan continues until either the end of the subject is  reached,  or
       there  are  no more unterminated paths. At this point, terminated paths
       represent the different matching possibilities (if there are none,  the
       match  has  failed).   Thus,  if there is more than one possible match,
       this algorithm finds all of them, and in particular, it finds the long-
       est.  The  matches are returned in decreasing order of length. There is
       an option to stop the algorithm after the first match (which is  neces-
       sarily the shortest) is found.

       Note that all the matches that are found start at the same point in the
       subject. If the pattern

         cat(er(pillar)?)?

       is matched against the string "the caterpillar catchment",  the  result
       will  be the three strings "caterpillar", "cater", and "cat" that start
       at the fifth character of the subject. The algorithm does not automati-
       cally move on to find matches that start at later positions.

       There are a number of features of PCRE regular expressions that are not
       supported by the alternative matching algorithm. They are as follows:

       1. Because the algorithm finds all  possible  matches,  the  greedy  or
       ungreedy  nature  of repetition quantifiers is not relevant. Greedy and
       ungreedy quantifiers are treated in exactly the same way. However, pos-
       sessive  quantifiers can make a difference when what follows could also
       match what is quantified, for example in a pattern like this:

         ^a++\w!

       This pattern matches "aaab!" but not "aaa!", which would be matched  by
       a  non-possessive quantifier. Similarly, if an atomic group is present,
       it is matched as if it were a standalone pattern at the current  point,
       and  the  longest match is then "locked in" for the rest of the overall
       pattern.

       2. When dealing with multiple paths through the tree simultaneously, it
       is  not  straightforward  to  keep track of captured substrings for the
       different matching possibilities, and  PCRE's  implementation  of  this
       algorithm does not attempt to do this. This means that no captured sub-
       strings are available.

       3. Because no substrings are captured, back references within the  pat-
       tern are not supported, and cause errors if encountered.

       4.  For  the same reason, conditional expressions that use a backrefer-
       ence as the condition or test for a specific group  recursion  are  not
       supported.

       5.  Because  many  paths  through the tree may be active, the \K escape
       sequence, which resets the start of the match when encountered (but may
       be  on  some  paths  and not on others), is not supported. It causes an
       error if encountered.

       6. Callouts are supported, but the value of the  capture_top  field  is
       always 1, and the value of the capture_last field is always -1.

       7.  The  \C  escape  sequence, which (in the standard algorithm) always
       matches a single data unit, even in UTF-8 or UTF-16 modes, is not  sup-
       ported  in these modes, because the alternative algorithm moves through
       the subject string one character (not data unit) at  a  time,  for  all
       active paths through the tree.

       8.  Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
       are not supported. (*FAIL) is supported, and  behaves  like  a  failing
       negative assertion.


ADVANTAGES OF THE ALTERNATIVE ALGORITHM

       Using  the alternative matching algorithm provides the following advan-
       tages:

       1. All possible matches (at a single point in the subject) are automat-
       ically  found,  and  in particular, the longest match is found. To find
       more than one match using the standard algorithm, you have to do kludgy
       things with callouts.

       2.  Because  the  alternative  algorithm  scans the subject string just
       once, and never needs to backtrack (except for lookbehinds), it is pos-
       sible  to  pass  very  long subject strings to the matching function in
       several pieces, checking for partial matching each time. Although it is
       possible  to  do multi-segment matching using the standard algorithm by
       retaining partially matched substrings, it  is  more  complicated.  The
       pcrepartial  documentation  gives  details of partial matching and dis-
       cusses multi-segment matching.


DISADVANTAGES OF THE ALTERNATIVE ALGORITHM

       The alternative algorithm suffers from a number of disadvantages:

       1. It is substantially slower than  the  standard  algorithm.  This  is
       partly  because  it has to search for all possible matches, but is also
       because it is less susceptible to optimization.

       2. Capturing parentheses and back references are not supported.

       3. Although atomic groups are supported, their use does not provide the
       performance advantage that it does for the standard algorithm.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 08 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREAPI(3)                                                          PCREAPI(3)


NAME
       PCRE - Perl-compatible regular expressions

       #include <pcre.h>


PCRE NATIVE API BASIC FUNCTIONS

       pcre *pcre_compile(const char *pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre_extra *pcre_study(const pcre *code, int options,
            const char **errptr);

       void pcre_free_study(pcre_extra *extra);

       int pcre_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);


PCRE NATIVE API STRING EXTRACTION FUNCTIONS

       int pcre_copy_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            char *buffer, int buffersize);

       int pcre_copy_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber, char *buffer,
            int buffersize);

       int pcre_get_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            const char **stringptr);

       int pcre_get_stringnumber(const pcre *code,
            const char *name);

       int pcre_get_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

       int pcre_get_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber,
            const char **stringptr);

       int pcre_get_substring_list(const char *subject,
            int *ovector, int stringcount, const char ***listptr);

       void pcre_free_substring(const char *stringptr);

       void pcre_free_substring_list(const char **stringptr);


PCRE NATIVE API AUXILIARY FUNCTIONS

       pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize);

       void pcre_jit_stack_free(pcre_jit_stack *stack);

       void pcre_assign_jit_stack(pcre_extra *extra,
            pcre_jit_callback callback, void *data);

       const unsigned char *pcre_maketables(void);

       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
            int what, void *where);

       int pcre_refcount(pcre *code, int adjust);

       int pcre_config(int what, void *where);

       const char *pcre_version(void);

       int pcre_pattern_to_host_byte_order(pcre *code,
            pcre_extra *extra, const unsigned char *tables);


PCRE NATIVE API INDIRECTED FUNCTIONS

       void *(*pcre_malloc)(size_t);

       void (*pcre_free)(void *);

       void *(*pcre_stack_malloc)(size_t);

       void (*pcre_stack_free)(void *);

       int (*pcre_callout)(pcre_callout_block *);


PCRE 8-BIT AND 16-BIT LIBRARIES

       From  release  8.30,  PCRE  can  be  compiled as a library for handling
       16-bit character strings as  well  as,  or  instead  of,  the  original
       library that handles 8-bit character strings. To avoid too much compli-
       cation, this document describes the 8-bit versions  of  the  functions,
       with only occasional references to the 16-bit library.

       The  16-bit  functions  operate in the same way as their 8-bit counter-
       parts; they just use different  data  types  for  their  arguments  and
       results, and their names start with pcre16_ instead of pcre_. For every
       option that has UTF8 in its name (for example, PCRE_UTF8), there  is  a
       corresponding 16-bit name with UTF8 replaced by UTF16. This facility is
       in fact just cosmetic; the 16-bit option names define the same bit val-
       ues.

       References to bytes and UTF-8 in this document should be read as refer-
       ences to 16-bit data  quantities  and  UTF-16  when  using  the  16-bit
       library,  unless specified otherwise. More details of the specific dif-
       ferences for the 16-bit library are given in the pcre16 page.


PCRE API OVERVIEW

       PCRE has its own native API, which is described in this document. There
       are  also some wrapper functions (for the 8-bit library only) that cor-
       respond to the POSIX regular expression  API,  but  they  do  not  give
       access  to  all  the functionality. They are described in the pcreposix
       documentation. Both of these APIs define a set of C function  calls.  A
       C++ wrapper (again for the 8-bit library only) is also distributed with
       PCRE. It is documented in the pcrecpp page.

       The native API C function prototypes are defined  in  the  header  file
       pcre.h,  and  on Unix-like systems the (8-bit) library itself is called
       libpcre. It can normally be accessed by adding -lpcre  to  the  command
       for  linking an application that uses PCRE. The header file defines the
       macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor release
       numbers  for the library. Applications can use these to include support
       for different releases of PCRE.

       In a Windows environment, if you want to statically link an application
       program  against  a  non-dll  pcre.a  file, you must define PCRE_STATIC
       before including pcre.h or pcrecpp.h, because otherwise  the  pcre_mal-
       loc()   and   pcre_free()   exported   functions   will   be   declared
       __declspec(dllimport), with unwanted results.

       The  functions  pcre_compile(),  pcre_compile2(),   pcre_study(),   and
       pcre_exec()  are used for compiling and matching regular expressions in
       a Perl-compatible manner. A sample program that demonstrates  the  sim-
       plest  way  of  using them is provided in the file called pcredemo.c in
       the PCRE source distribution. A listing of this program is given in the
       pcredemo  documentation, and the pcresample documentation describes how
       to compile and run it.

       Just-in-time compiler support is an optional feature of PCRE  that  can
       be built in appropriate hardware environments. It greatly speeds up the
       matching performance of  many  patterns.  Simple  programs  can  easily
       request  that  it  be  used  if available, by setting an option that is
       ignored when it is not relevant. More complicated programs  might  need
       to     make    use    of    the    functions    pcre_jit_stack_alloc(),
       pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to  control
       the  JIT  code's  memory  usage.   These functions are discussed in the
       pcrejit documentation.

       A second matching function, pcre_dfa_exec(), which is not Perl-compati-
       ble,  is  also provided. This uses a different algorithm for the match-
       ing. The alternative algorithm finds all possible matches (at  a  given
       point  in  the  subject), and scans the subject just once (unless there
       are lookbehind assertions). However, this  algorithm  does  not  return
       captured  substrings.  A description of the two matching algorithms and
       their advantages and disadvantages is given in the  pcrematching  docu-
       mentation.

       In  addition  to  the  main compiling and matching functions, there are
       convenience functions for extracting captured substrings from a subject
       string that is matched by pcre_exec(). They are:

         pcre_copy_substring()
         pcre_copy_named_substring()
         pcre_get_substring()
         pcre_get_named_substring()
         pcre_get_substring_list()
         pcre_get_stringnumber()
         pcre_get_stringtable_entries()

       pcre_free_substring() and pcre_free_substring_list() are also provided,
       to free the memory used for extracted strings.

       The function pcre_maketables() is used to  build  a  set  of  character
       tables   in   the   current   locale  for  passing  to  pcre_compile(),
       pcre_exec(), or pcre_dfa_exec(). This is an optional facility  that  is
       provided  for  specialist  use.  Most  commonly,  no special tables are
       passed, in which case internal tables that are generated when  PCRE  is
       built are used.

       The  function  pcre_fullinfo()  is used to find out information about a
       compiled pattern. The function pcre_version() returns a  pointer  to  a
       string containing the version of PCRE and its date of release.

       The  function  pcre_refcount()  maintains  a  reference count in a data
       block containing a compiled pattern. This is provided for  the  benefit
       of object-oriented applications.

       The  global  variables  pcre_malloc and pcre_free initially contain the
       entry points of the standard malloc()  and  free()  functions,  respec-
       tively. PCRE calls the memory management functions via these variables,
       so a calling program can replace them if it  wishes  to  intercept  the
       calls. This should be done before calling any PCRE functions.

       The  global  variables  pcre_stack_malloc  and pcre_stack_free are also
       indirections to memory management functions.  These  special  functions
       are  used  only  when  PCRE is compiled to use the heap for remembering
       data, instead of recursive function calls, when running the pcre_exec()
       function.  See  the  pcrebuild  documentation  for details of how to do
       this. It is a non-standard way of building PCRE, for  use  in  environ-
       ments  that  have  limited stacks. Because of the greater use of memory
       management, it runs more slowly. Separate  functions  are  provided  so
       that  special-purpose  external  code  can  be used for this case. When
       used, these functions are always called in a  stack-like  manner  (last
       obtained,  first freed), and always for memory blocks of the same size.
       There is a discussion about PCRE's stack usage in the  pcrestack  docu-
       mentation.

       The global variable pcre_callout initially contains NULL. It can be set
       by the caller to a "callout" function, which PCRE  will  then  call  at
       specified  points during a matching operation. Details are given in the
       pcrecallout documentation.


NEWLINES

       PCRE supports five different conventions for indicating line breaks  in
       strings:  a  single  CR (carriage return) character, a single LF (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding,  or any Unicode newline sequence. The Unicode newline sequences
       are the three just mentioned, plus the single characters  VT  (vertical
       tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line
       separator, U+2028), and PS (paragraph separator, U+2029).

       Each of the first three conventions is used by at least  one  operating
       system  as its standard newline sequence. When PCRE is built, a default
       can be specified.  The default default is LF, which is the  Unix  stan-
       dard.  When  PCRE  is run, the default can be overridden, either when a
       pattern is compiled, or when it is matched.

       At compile time, the newline convention can be specified by the options
       argument  of  pcre_compile(), or it can be specified by special text at
       the start of the pattern itself; this overrides any other settings. See
       the pcrepattern page for details of the special character sequences.

       In the PCRE documentation the word "newline" is used to mean "the char-
       acter or pair of characters that indicate a line break". The choice  of
       newline  convention  affects  the  handling of the dot, circumflex, and
       dollar metacharacters, the handling of #-comments in /x mode, and, when
       CRLF  is a recognized line ending sequence, the match position advance-
       ment for a non-anchored pattern. There is more detail about this in the
       section on pcre_exec() options below.

       The  choice of newline convention does not affect the interpretation of
       the \n or \r escape sequences, nor does  it  affect  what  \R  matches,
       which is controlled in a similar way, but by separate options.


MULTITHREADING

       The  PCRE  functions  can be used in multi-threading applications, with
       the  proviso  that  the  memory  management  functions  pointed  to  by
       pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
       callout function pointed to by pcre_callout, are shared by all threads.

       The compiled form of a regular expression is not altered during  match-
       ing, so the same compiled pattern can safely be used by several threads
       at once.

       If the just-in-time optimization feature is being used, it needs  sepa-
       rate  memory stack areas for each thread. See the pcrejit documentation
       for more details.


SAVING PRECOMPILED PATTERNS FOR LATER USE

       The compiled form of a regular expression can be saved and re-used at a
       later  time,  possibly by a different program, and even on a host other
       than the one on which  it  was  compiled.  Details  are  given  in  the
       pcreprecompile  documentation,  which  includes  a  description  of the
       pcre_pattern_to_host_byte_order() function. However, compiling a  regu-
       lar  expression  with one version of PCRE for use with a different ver-
       sion is not guaranteed to work and may cause crashes.


CHECKING BUILD-TIME OPTIONS

       int pcre_config(int what, void *where);

       The function pcre_config() makes it possible for a PCRE client to  dis-
       cover which optional features have been compiled into the PCRE library.
       The pcrebuild documentation has more details about these optional  fea-
       tures.

       The  first  argument  for pcre_config() is an integer, specifying which
       information is required; the second argument is a pointer to a variable
       into  which  the  information  is placed. The returned value is zero on
       success, or the negative error code PCRE_ERROR_BADOPTION if  the  value
       in  the  first argument is not recognized. The following information is
       available:

         PCRE_CONFIG_UTF8

       The output is an integer that is set to one if UTF-8 support is  avail-
       able;  otherwise  it  is  set  to  zero. If this option is given to the
       16-bit  version  of  this  function,  pcre16_config(),  the  result  is
       PCRE_ERROR_BADOPTION.

         PCRE_CONFIG_UTF16

       The output is an integer that is set to one if UTF-16 support is avail-
       able; otherwise it is set to zero. This value should normally be  given
       to the 16-bit version of this function, pcre16_config(). If it is given
       to the 8-bit version of this function, the result is  PCRE_ERROR_BADOP-
       TION.

         PCRE_CONFIG_UNICODE_PROPERTIES

       The  output  is  an  integer  that is set to one if support for Unicode
       character properties is available; otherwise it is set to zero.

         PCRE_CONFIG_JIT

       The output is an integer that is set to one if support for just-in-time
       compiling is available; otherwise it is set to zero.

         PCRE_CONFIG_JITTARGET

       The  output is a pointer to a zero-terminated "const char *" string. If
       JIT support is available, the string contains the name of the architec-
       ture  for  which the JIT compiler is configured, for example "x86 32bit
       (little endian + unaligned)". If JIT  support  is  not  available,  the
       result is NULL.

         PCRE_CONFIG_NEWLINE

       The  output  is  an integer whose value specifies the default character
       sequence that is recognized as meaning "newline". The four values  that
       are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
       and -1 for ANY.  Though they are derived from ASCII,  the  same  values
       are returned in EBCDIC environments. The default should normally corre-
       spond to the standard sequence for your operating system.

         PCRE_CONFIG_BSR

       The output is an integer whose value indicates what character sequences
       the  \R  escape sequence matches by default. A value of 0 means that \R
       matches any Unicode line ending sequence; a value of 1  means  that  \R
       matches only CR, LF, or CRLF. The default can be overridden when a pat-
       tern is compiled or matched.

         PCRE_CONFIG_LINK_SIZE

       The output is an integer that contains the number  of  bytes  used  for
       internal  linkage  in  compiled  regular  expressions.  For  the  8-bit
       library, the value can be 2, 3, or 4. For the 16-bit library, the value
       is either 2 or 4 and is still a number of bytes. The default value of 2
       is sufficient for all but the most massive patterns,  since  it  allows
       the  compiled  pattern  to  be  up to 64K in size.  Larger values allow
       larger regular expressions to be compiled, at  the  expense  of  slower
       matching.

         PCRE_CONFIG_POSIX_MALLOC_THRESHOLD

       The  output  is  an integer that contains the threshold above which the
       POSIX interface uses malloc() for output vectors. Further  details  are
       given in the pcreposix documentation.

         PCRE_CONFIG_MATCH_LIMIT

       The  output is a long integer that gives the default limit for the num-
       ber of internal matching function calls  in  a  pcre_exec()  execution.
       Further details are given with pcre_exec() below.

         PCRE_CONFIG_MATCH_LIMIT_RECURSION

       The output is a long integer that gives the default limit for the depth
       of  recursion  when  calling  the  internal  matching  function  in   a
       pcre_exec()  execution.  Further  details  are  given  with pcre_exec()
       below.

         PCRE_CONFIG_STACKRECURSE

       The output is an integer that is set to one if internal recursion  when
       running pcre_exec() is implemented by recursive function calls that use
       the stack to remember their state. This is the usual way that  PCRE  is
       compiled. The output is zero if PCRE was compiled to use blocks of data
       on the  heap  instead  of  recursive  function  calls.  In  this  case,
       pcre_stack_malloc  and  pcre_stack_free  are  called  to  manage memory
       blocks on the heap, thus avoiding the use of the stack.


COMPILING A PATTERN

       pcre *pcre_compile(const char *pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       Either of the functions pcre_compile() or pcre_compile2() can be called
       to compile a pattern into an internal form. The only difference between
       the two interfaces is that pcre_compile2() has an additional  argument,
       errorcodeptr,  via  which  a  numerical  error code can be returned. To
       avoid too much repetition, we refer just to pcre_compile()  below,  but
       the information applies equally to pcre_compile2().

       The pattern is a C string terminated by a binary zero, and is passed in
       the pattern argument. A pointer to a single block  of  memory  that  is
       obtained  via  pcre_malloc is returned. This contains the compiled code
       and related data. The pcre type is defined for the returned block; this
       is a typedef for a structure whose contents are not externally defined.
       It is up to the caller to free the memory (via pcre_free) when it is no
       longer required.

       Although  the compiled code of a PCRE regex is relocatable, that is, it
       does not depend on memory location, the complete pcre data block is not
       fully  relocatable, because it may contain a copy of the tableptr argu-
       ment, which is an address (see below).

       The options argument contains various bit settings that affect the com-
       pilation.  It  should be zero if no options are required. The available
       options are described below. Some of them (in  particular,  those  that
       are  compatible with Perl, but some others as well) can also be set and
       unset from within the pattern (see  the  detailed  description  in  the
       pcrepattern  documentation). For those options that can be different in
       different parts of the pattern, the contents of  the  options  argument
       specifies their settings at the start of compilation and execution. The
       PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK,  and
       PCRE_NO_START_OPTIMIZE  options  can  be set at the time of matching as
       well as at compile time.

       If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
       if  compilation  of  a  pattern fails, pcre_compile() returns NULL, and
       sets the variable pointed to by errptr to point to a textual error mes-
       sage. This is a static string that is part of the library. You must not
       try to free it. Normally, the offset from the start of the  pattern  to
       the  byte  that  was  being  processed when the error was discovered is
       placed in the variable pointed to by erroffset, which must not be  NULL
       (if  it is, an immediate error is given). However, for an invalid UTF-8
       string, the offset is that of the first byte of the failing character.

       Some errors are not detected until the whole pattern has been  scanned;
       in  these  cases,  the offset passed back is the length of the pattern.
       Note that the offset is in bytes, not characters, even in  UTF-8  mode.
       It may sometimes point into the middle of a UTF-8 character.

       If  pcre_compile2()  is  used instead of pcre_compile(), and the error-
       codeptr argument is not NULL, a non-zero error code number is  returned
       via  this argument in the event of an error. This is in addition to the
       textual error message. Error codes and messages are listed below.

       If the final argument, tableptr, is NULL, PCRE uses a  default  set  of
       character  tables  that  are  built  when  PCRE  is compiled, using the
       default C locale. Otherwise, tableptr must be an address  that  is  the
       result  of  a  call to pcre_maketables(). This value is stored with the
       compiled pattern, and used again by pcre_exec(), unless  another  table
       pointer is passed to it. For more discussion, see the section on locale
       support below.

       This code fragment shows a typical straightforward  call  to  pcre_com-
       pile():

         pcre *re;
         const char *error;
         int erroffset;
         re = pcre_compile(
           "^A.*Z",          /* the pattern */
           0,                /* default options */
           &error,           /* for error message */
           &erroffset,       /* for error offset */
           NULL);            /* use default character tables */

       The  following  names  for option bits are defined in the pcre.h header
       file:

         PCRE_ANCHORED

       If this bit is set, the pattern is forced to be "anchored", that is, it
       is  constrained to match only at the first matching point in the string
       that is being searched (the "subject string"). This effect can also  be
       achieved  by appropriate constructs in the pattern itself, which is the
       only way to do it in Perl.

         PCRE_AUTO_CALLOUT

       If this bit is set, pcre_compile() automatically inserts callout items,
       all  with  number  255, before each pattern item. For discussion of the
       callout facility, see the pcrecallout documentation.

         PCRE_BSR_ANYCRLF
         PCRE_BSR_UNICODE

       These options (which are mutually exclusive) control what the \R escape
       sequence  matches.  The choice is either to match only CR, LF, or CRLF,
       or to match any Unicode newline sequence. The default is specified when
       PCRE is built. It can be overridden from within the pattern, or by set-
       ting an option when a compiled pattern is matched.

         PCRE_CASELESS

       If this bit is set, letters in the pattern match both upper  and  lower
       case  letters.  It  is  equivalent  to  Perl's /i option, and it can be
       changed within a pattern by a (?i) option setting. In UTF-8 mode,  PCRE
       always  understands the concept of case for characters whose values are
       less than 128, so caseless matching is always possible. For  characters
       with  higher  values,  the concept of case is supported if PCRE is com-
       piled with Unicode property support, but not otherwise. If you want  to
       use  caseless  matching  for  characters 128 and above, you must ensure
       that PCRE is compiled with Unicode property support  as  well  as  with
       UTF-8 support.

         PCRE_DOLLAR_ENDONLY

       If  this bit is set, a dollar metacharacter in the pattern matches only
       at the end of the subject string. Without this option,  a  dollar  also
       matches  immediately before a newline at the end of the string (but not
       before any other newlines). The PCRE_DOLLAR_ENDONLY option  is  ignored
       if  PCRE_MULTILINE  is  set.   There is no equivalent to this option in
       Perl, and no way to set it within a pattern.

         PCRE_DOTALL

       If this bit is set, a dot metacharacter in the pattern matches a  char-
       acter of any value, including one that indicates a newline. However, it
       only ever matches one character, even if newlines are  coded  as  CRLF.
       Without  this option, a dot does not match when the current position is
       at a newline. This option is equivalent to Perl's /s option, and it can
       be  changed within a pattern by a (?s) option setting. A negative class
       such as [^a] always matches newline characters, independent of the set-
       ting of this option.

         PCRE_DUPNAMES

       If  this  bit is set, names used to identify capturing subpatterns need
       not be unique. This can be helpful for certain types of pattern when it
       is  known  that  only  one instance of the named subpattern can ever be
       matched. There are more details of named subpatterns  below;  see  also
       the pcrepattern documentation.

         PCRE_EXTENDED

       If  this  bit  is  set,  white space data characters in the pattern are
       totally ignored except when escaped or inside a character class.  White
       space does not include the VT character (code 11). In addition, charac-
       ters between an unescaped # outside a character class and the next new-
       line,  inclusive,  are  also  ignored.  This is equivalent to Perl's /x
       option, and it can be changed within a pattern by a  (?x)  option  set-
       ting.

       Which  characters  are  interpreted  as  newlines  is controlled by the
       options passed to pcre_compile() or by a special sequence at the  start
       of  the  pattern, as described in the section entitled "Newline conven-
       tions" in the pcrepattern documentation. Note that the end of this type
       of  comment  is  a  literal  newline  sequence  in  the pattern; escape
       sequences that happen to represent a newline do not count.

       This option makes it possible to include  comments  inside  complicated
       patterns.   Note,  however,  that this applies only to data characters.
       White space  characters  may  never  appear  within  special  character
       sequences in a pattern, for example within the sequence (?( that intro-
       duces a conditional subpattern.

         PCRE_EXTRA

       This option was invented in order to turn on  additional  functionality
       of  PCRE  that  is  incompatible with Perl, but it is currently of very
       little use. When set, any backslash in a pattern that is followed by  a
       letter  that  has  no  special  meaning causes an error, thus reserving
       these combinations for future expansion. By  default,  as  in  Perl,  a
       backslash  followed by a letter with no special meaning is treated as a
       literal. (Perl can, however, be persuaded to give an error for this, by
       running  it with the -w option.) There are at present no other features
       controlled by this option. It can also be set by a (?X) option  setting
       within a pattern.

         PCRE_FIRSTLINE

       If  this  option  is  set,  an  unanchored pattern is required to match
       before or at the first  newline  in  the  subject  string,  though  the
       matched text may continue over the newline.

         PCRE_JAVASCRIPT_COMPAT

       If this option is set, PCRE's behaviour is changed in some ways so that
       it is compatible with JavaScript rather than Perl. The changes  are  as
       follows:

       (1)  A  lone  closing square bracket in a pattern causes a compile-time
       error, because this is illegal in JavaScript (by default it is  treated
       as a data character). Thus, the pattern AB]CD becomes illegal when this
       option is set.

       (2) At run time, a back reference to an unset subpattern group  matches
       an  empty  string (by default this causes the current matching alterna-
       tive to fail). A pattern such as (\1)(a) succeeds when this  option  is
       set  (assuming  it can find an "a" in the subject), whereas it fails by
       default, for Perl compatibility.

       (3) \U matches an upper case "U" character; by default \U causes a com-
       pile time error (Perl uses \U to upper case subsequent characters).

       (4) \u matches a lower case "u" character unless it is followed by four
       hexadecimal digits, in which case the hexadecimal  number  defines  the
       code  point  to match. By default, \u causes a compile time error (Perl
       uses it to upper case the following character).

       (5) \x matches a lower case "x" character unless it is followed by  two
       hexadecimal  digits,  in  which case the hexadecimal number defines the
       code point to match. By default, as in Perl, a  hexadecimal  number  is
       always expected after \x, but it may have zero, one, or two digits (so,
       for example, \xz matches a binary zero character followed by z).

         PCRE_MULTILINE

       By default, PCRE treats the subject string as consisting  of  a  single
       line  of characters (even if it actually contains newlines). The "start
       of line" metacharacter (^) matches only at the  start  of  the  string,
       while  the  "end  of line" metacharacter ($) matches only at the end of
       the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
       is set). This is the same as Perl.

       When  PCRE_MULTILINE  it  is set, the "start of line" and "end of line"
       constructs match immediately following or immediately  before  internal
       newlines  in  the  subject string, respectively, as well as at the very
       start and end. This is equivalent to Perl's /m option, and  it  can  be
       changed within a pattern by a (?m) option setting. If there are no new-
       lines in a subject string, or no occurrences of ^ or $  in  a  pattern,
       setting PCRE_MULTILINE has no effect.

         PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

       These  options  override the default newline definition that was chosen
       when PCRE was built. Setting the first or the second specifies  that  a
       newline  is  indicated  by a single character (CR or LF, respectively).
       Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by  the
       two-character  CRLF  sequence.  Setting  PCRE_NEWLINE_ANYCRLF specifies
       that any of the three preceding sequences should be recognized. Setting
       PCRE_NEWLINE_ANY  specifies that any Unicode newline sequence should be
       recognized. The Unicode newline sequences are the three just mentioned,
       plus  the  single  characters VT (vertical tab, U+000B), FF (form feed,
       U+000C), NEL (next line, U+0085), LS (line separator, U+2028),  and  PS
       (paragraph  separator, U+2029). For the 8-bit library, the last two are
       recognized only in UTF-8 mode.

       The newline setting in the  options  word  uses  three  bits  that  are
       treated as a number, giving eight possibilities. Currently only six are
       used (default plus the five values above). This means that if  you  set
       more  than one newline option, the combination may or may not be sensi-
       ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
       PCRE_NEWLINE_CRLF,  but other combinations may yield unused numbers and
       cause an error.

       The only time that a line break in a pattern  is  specially  recognized
       when  compiling is when PCRE_EXTENDED is set. CR and LF are white space
       characters, and so are ignored in this mode. Also, an unescaped #  out-
       side  a  character class indicates a comment that lasts until after the
       next line break sequence. In other circumstances, line break  sequences
       in patterns are treated as literal data.

       The newline option that is set at compile time becomes the default that
       is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.

         PCRE_NO_AUTO_CAPTURE

       If this option is set, it disables the use of numbered capturing paren-
       theses  in the pattern. Any opening parenthesis that is not followed by
       ? behaves as if it were followed by ?: but named parentheses can  still
       be  used  for  capturing  (and  they acquire numbers in the usual way).
       There is no equivalent of this option in Perl.

         NO_START_OPTIMIZE

       This is an option that acts at matching time; that is, it is really  an
       option  for  pcre_exec()  or  pcre_dfa_exec().  If it is set at compile
       time, it is remembered with the compiled pattern and assumed at  match-
       ing  time.  For  details  see  the discussion of PCRE_NO_START_OPTIMIZE
       below.

         PCRE_UCP

       This option changes the way PCRE processes \B, \b, \D, \d, \S, \s,  \W,
       \w,  and  some  of  the POSIX character classes. By default, only ASCII
       characters are recognized, but if PCRE_UCP is set,  Unicode  properties
       are  used instead to classify characters. More details are given in the
       section on generic character types in the pcrepattern page. If you  set
       PCRE_UCP,  matching  one of the items it affects takes much longer. The
       option is available only if PCRE has been compiled with  Unicode  prop-
       erty support.

         PCRE_UNGREEDY

       This  option  inverts  the "greediness" of the quantifiers so that they
       are not greedy by default, but become greedy if followed by "?". It  is
       not  compatible  with Perl. It can also be set by a (?U) option setting
       within the pattern.

         PCRE_UTF8

       This option causes PCRE to regard both the pattern and the  subject  as
       strings of UTF-8 characters instead of single-byte strings. However, it
       is available only when PCRE is built to include UTF  support.  If  not,
       the  use  of  this option provokes an error. Details of how this option
       changes the behaviour of PCRE are given in the pcreunicode page.

         PCRE_NO_UTF8_CHECK

       When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
       automatically  checked.  There  is  a  discussion about the validity of
       UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence  is
       found,  pcre_compile()  returns an error. If you already know that your
       pattern is valid, and you want to skip this check for performance  rea-
       sons,  you  can set the PCRE_NO_UTF8_CHECK option.  When it is set, the
       effect of passing an invalid UTF-8 string as a pattern is undefined. It
       may  cause  your  program  to  crash. Note that this option can also be
       passed to pcre_exec() and pcre_dfa_exec(),  to  suppress  the  validity
       checking of subject strings.


COMPILATION ERROR CODES

       The  following  table  lists  the  error  codes than may be returned by
       pcre_compile2(), along with the error messages that may be returned  by
       both  compiling  functions.  Note  that error messages are always 8-bit
       ASCII strings, even in 16-bit mode. As PCRE has developed,  some  error
       codes  have  fallen  out of use. To avoid confusion, they have not been
       re-used.

          0  no error
          1  \ at end of pattern
          2  \c at end of pattern
          3  unrecognized character follows \
          4  numbers out of order in {} quantifier
          5  number too big in {} quantifier
          6  missing terminating ] for character class
          7  invalid escape sequence in character class
          8  range out of order in character class
          9  nothing to repeat
         10  [this code is not in use]
         11  internal error: unexpected repeat
         12  unrecognized character after (? or (?-
         13  POSIX named classes are supported only within a class
         14  missing )
         15  reference to non-existent subpattern
         16  erroffset passed as NULL
         17  unknown option bit(s) set
         18  missing ) after comment
         19  [this code is not in use]
         20  regular expression is too large
         21  failed to get memory
         22  unmatched parentheses
         23  internal error: code overflow
         24  unrecognized character after (?<
         25  lookbehind assertion is not fixed length
         26  malformed number or name after (?(
         27  conditional group contains more than two branches
         28  assertion expected after (?(
         29  (?R or (?[+-]digits must be followed by )
         30  unknown POSIX class name
         31  POSIX collating elements are not supported
         32  this version of PCRE is compiled without UTF support
         33  [this code is not in use]
         34  character value in \x{...} sequence is too large
         35  invalid condition (?(0)
         36  \C not allowed in lookbehind assertion
         37  PCRE does not support \L, \l, \N{name}, \U, or \u
         38  number after (?C is > 255
         39  closing ) for (?C expected
         40  recursive call could loop indefinitely
         41  unrecognized character after (?P
         42  syntax error in subpattern name (missing terminator)
         43  two named subpatterns have the same name
         44  invalid UTF-8 string (specifically UTF-8)
         45  support for \P, \p, and \X has not been compiled
         46  malformed \P or \p sequence
         47  unknown property name after \P or \p
         48  subpattern name is too long (maximum 32 characters)
         49  too many named subpatterns (maximum 10000)
         50  [this code is not in use]
         51  octal value is greater than \377 in 8-bit non-UTF-8 mode
         52  internal error: overran compiling workspace
         53  internal error: previously-checked referenced subpattern
               not found
         54  DEFINE group contains more than one branch
         55  repeating a DEFINE group is not allowed
         56  inconsistent NEWLINE options
         57  \g is not followed by a braced, angle-bracketed, or quoted
               name/number or by a plain number
         58  a numbered reference must not be zero
         59  an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
         60  (*VERB) not recognized
         61  number is too big
         62  subpattern name expected
         63  digit expected after (?+
         64  ] is an invalid data character in JavaScript compatibility mode
         65  different names for subpatterns of the same number are
               not allowed
         66  (*MARK) must have an argument
         67  this version of PCRE is not compiled with Unicode property
               support
         68  \c must be followed by an ASCII character
         69  \k is not followed by a braced, angle-bracketed, or quoted name
         70  internal error: unknown opcode in find_fixedlength()
         71  \N is not supported in a class
         72  too many forward references
         73  disallowed Unicode code point (>= 0xd800 && <= 0xdfff)
         74  invalid UTF-16 string (specifically UTF-16)
         75  name is too long in (*MARK), (*PRUNE), (*SKIP), or (*THEN)

       The numbers 32 and 10000 in errors 48 and 49  are  defaults;  different
       values may be used if the limits were changed when PCRE was built.


STUDYING A PATTERN

       pcre_extra *pcre_study(const pcre *code, int options
            const char **errptr);

       If  a  compiled  pattern is going to be used several times, it is worth
       spending more time analyzing it in order to speed up the time taken for
       matching.  The function pcre_study() takes a pointer to a compiled pat-
       tern as its first argument. If studying the pattern produces additional
       information  that  will  help speed up matching, pcre_study() returns a
       pointer to a pcre_extra block, in which the study_data field points  to
       the results of the study.

       The  returned  value  from  pcre_study()  can  be  passed  directly  to
       pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block  also  con-
       tains  other  fields  that can be set by the caller before the block is
       passed; these are described below in the section on matching a pattern.

       If studying the  pattern  does  not  produce  any  useful  information,
       pcre_study() returns NULL. In that circumstance, if the calling program
       wants  to  pass  any  of   the   other   fields   to   pcre_exec()   or
       pcre_dfa_exec(), it must set up its own pcre_extra block.

       The  second  argument  of  pcre_study() contains option bits. There are
       three options:

         PCRE_STUDY_JIT_COMPILE
         PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
         PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE

       If any of these are set, and the just-in-time  compiler  is  available,
       the  pattern  is  further compiled into machine code that executes much
       faster than the pcre_exec()  interpretive  matching  function.  If  the
       just-in-time  compiler is not available, these options are ignored. All
       other bits in the options argument must be zero.

       JIT compilation is a heavyweight optimization. It can  take  some  time
       for  patterns  to  be analyzed, and for one-off matches and simple pat-
       terns the benefit of faster execution might be offset by a much  slower
       study time.  Not all patterns can be optimized by the JIT compiler. For
       those that cannot be handled, matching automatically falls back to  the
       pcre_exec()  interpreter.  For more details, see the pcrejit documenta-
       tion.

       The third argument for pcre_study() is a pointer for an error  message.
       If  studying  succeeds  (even  if no data is returned), the variable it
       points to is set to NULL. Otherwise it is set to  point  to  a  textual
       error message. This is a static string that is part of the library. You
       must not try to free it. You should test the  error  pointer  for  NULL
       after calling pcre_study(), to be sure that it has run successfully.

       When  you are finished with a pattern, you can free the memory used for
       the study data by calling pcre_free_study(). This function was added to
       the  API  for  release  8.20. For earlier versions, the memory could be
       freed with pcre_free(), just like the pattern itself. This  will  still
       work  in  cases where JIT optimization is not used, but it is advisable
       to change to the new function when convenient.

       This is a typical way in which pcre_study() is used (except that  in  a
       real application there should be tests for errors):

         int rc;
         pcre *re;
         pcre_extra *sd;
         re = pcre_compile("pattern", 0, &error, &erroroffset, NULL);
         sd = pcre_study(
           re,             /* result of pcre_compile() */
           0,              /* no options */
           &error);        /* set to NULL or points to a message */
         rc = pcre_exec(   /* see below for details of pcre_exec() options */
           re, sd, "subject", 7, 0, 0, ovector, 30);
         ...
         pcre_free_study(sd);
         pcre_free(re);

       Studying a pattern does two things: first, a lower bound for the length
       of subject string that is needed to match the pattern is computed. This
       does not mean that there are any strings of that length that match, but
       it does guarantee that no shorter strings match. The value is  used  by
       pcre_exec()  and  pcre_dfa_exec()  to  avoid  wasting time by trying to
       match strings that are shorter than the lower bound. You can  find  out
       the value in a calling program via the pcre_fullinfo() function.

       Studying a pattern is also useful for non-anchored patterns that do not
       have a single fixed starting character. A bitmap of  possible  starting
       bytes  is  created. This speeds up finding a position in the subject at
       which to start matching. (In 16-bit mode, the bitmap is used for 16-bit
       values less than 256.)

       These  two optimizations apply to both pcre_exec() and pcre_dfa_exec(),
       and the information is also used by the JIT  compiler.   The  optimiza-
       tions can be disabled by setting the PCRE_NO_START_OPTIMIZE option when
       calling pcre_exec() or pcre_dfa_exec(), but if this is done, JIT execu-
       tion  is  also disabled. You might want to do this if your pattern con-
       tains callouts or (*MARK) and you want to make use of these  facilities
       in    cases    where    matching   fails.   See   the   discussion   of
       PCRE_NO_START_OPTIMIZE below.


LOCALE SUPPORT

       PCRE handles caseless matching, and determines whether  characters  are
       letters,  digits, or whatever, by reference to a set of tables, indexed
       by character value. When running in UTF-8 mode, this  applies  only  to
       characters  with  codes  less than 128. By default, higher-valued codes
       never match escapes such as \w or \d, but they can be tested with \p if
       PCRE  is  built with Unicode character property support. Alternatively,
       the PCRE_UCP option can be set at compile  time;  this  causes  \w  and
       friends to use Unicode property support instead of built-in tables. The
       use of locales with Unicode is discouraged. If you are handling charac-
       ters  with codes greater than 128, you should either use UTF-8 and Uni-
       code, or use locales, but not try to mix the two.

       PCRE contains an internal set of tables that are used  when  the  final
       argument  of  pcre_compile()  is  NULL.  These  are sufficient for many
       applications.  Normally, the internal tables recognize only ASCII char-
       acters. However, when PCRE is built, it is possible to cause the inter-
       nal tables to be rebuilt in the default "C" locale of the local system,
       which may cause them to be different.

       The  internal tables can always be overridden by tables supplied by the
       application that calls PCRE. These may be created in a different locale
       from  the  default.  As more and more applications change to using Uni-
       code, the need for this locale support is expected to die away.

       External tables are built by calling  the  pcre_maketables()  function,
       which  has no arguments, in the relevant locale. The result can then be
       passed to pcre_compile() or pcre_exec()  as  often  as  necessary.  For
       example,  to  build  and use tables that are appropriate for the French
       locale (where accented characters with  values  greater  than  128  are
       treated as letters), the following code could be used:

         setlocale(LC_CTYPE, "fr_FR");
         tables = pcre_maketables();
         re = pcre_compile(..., tables);

       The  locale  name "fr_FR" is used on Linux and other Unix-like systems;
       if you are using Windows, the name for the French locale is "french".

       When pcre_maketables() runs, the tables are built  in  memory  that  is
       obtained  via  pcre_malloc. It is the caller's responsibility to ensure
       that the memory containing the tables remains available for as long  as
       it is needed.

       The pointer that is passed to pcre_compile() is saved with the compiled
       pattern, and the same tables are used via this pointer by  pcre_study()
       and normally also by pcre_exec(). Thus, by default, for any single pat-
       tern, compilation, studying and matching all happen in the same locale,
       but different patterns can be compiled in different locales.

       It  is  possible to pass a table pointer or NULL (indicating the use of
       the internal tables) to pcre_exec(). Although  not  intended  for  this
       purpose,  this facility could be used to match a pattern in a different
       locale from the one in which it was compiled. Passing table pointers at
       run time is discussed below in the section on matching a pattern.


INFORMATION ABOUT A PATTERN

       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
            int what, void *where);

       The  pcre_fullinfo() function returns information about a compiled pat-
       tern. It replaces the pcre_info() function, which was removed from  the
       library at version 8.30, after more than 10 years of obsolescence.

       The  first  argument  for  pcre_fullinfo() is a pointer to the compiled
       pattern. The second argument is the result of pcre_study(), or NULL  if
       the  pattern  was not studied. The third argument specifies which piece
       of information is required, and the fourth argument is a pointer  to  a
       variable  to  receive  the  data. The yield of the function is zero for
       success, or one of the following negative numbers:

         PCRE_ERROR_NULL           the argument code was NULL
                                   the argument where was NULL
         PCRE_ERROR_BADMAGIC       the "magic number" was not found
         PCRE_ERROR_BADENDIANNESS  the pattern was compiled with different
                                   endianness
         PCRE_ERROR_BADOPTION      the value of what was invalid

       The "magic number" is placed at the start of each compiled  pattern  as
       an  simple check against passing an arbitrary memory pointer. The endi-
       anness error can occur if a compiled pattern is saved and reloaded on a
       different  host.  Here  is a typical call of pcre_fullinfo(), to obtain
       the length of the compiled pattern:

         int rc;
         size_t length;
         rc = pcre_fullinfo(
           re,               /* result of pcre_compile() */
           sd,               /* result of pcre_study(), or NULL */
           PCRE_INFO_SIZE,   /* what is required */
           &length);         /* where to put the data */

       The possible values for the third argument are defined in  pcre.h,  and
       are as follows:

         PCRE_INFO_BACKREFMAX

       Return  the  number  of  the highest back reference in the pattern. The
       fourth argument should point to an int variable. Zero  is  returned  if
       there are no back references.

         PCRE_INFO_CAPTURECOUNT

       Return  the  number of capturing subpatterns in the pattern. The fourth
       argument should point to an int variable.

         PCRE_INFO_DEFAULT_TABLES

       Return a pointer to the internal default character tables within  PCRE.
       The  fourth  argument should point to an unsigned char * variable. This
       information call is provided for internal use by the pcre_study() func-
       tion.  External  callers  can  cause PCRE to use its internal tables by
       passing a NULL table pointer.

         PCRE_INFO_FIRSTBYTE

       Return information about the first data unit of any matched string, for
       a  non-anchored  pattern.  (The name of this option refers to the 8-bit
       library, where data units are bytes.) The fourth argument should  point
       to an int variable.

       If  there  is  a  fixed first value, for example, the letter "c" from a
       pattern such as (cat|cow|coyote), its value is returned. In  the  8-bit
       library,  the  value is always less than 256; in the 16-bit library the
       value can be up to 0xffff.

       If there is no fixed first value, and if either

       (a) the pattern was compiled with the PCRE_MULTILINE option, and  every
       branch starts with "^", or

       (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
       set (if it were set, the pattern would be anchored),

       -1 is returned, indicating that the pattern matches only at  the  start
       of  a  subject string or after any newline within the string. Otherwise
       -2 is returned. For anchored patterns, -2 is returned.

         PCRE_INFO_FIRSTTABLE

       If the pattern was studied, and this resulted in the construction of  a
       256-bit  table indicating a fixed set of values for the first data unit
       in any matching string, a pointer to the table is  returned.  Otherwise
       NULL  is returned. The fourth argument should point to an unsigned char
       * variable.

         PCRE_INFO_HASCRORLF

       Return 1 if the pattern contains any explicit  matches  for  CR  or  LF
       characters,  otherwise  0.  The  fourth argument should point to an int
       variable. An explicit match is either a literal CR or LF character,  or
       \r or \n.

         PCRE_INFO_JCHANGED

       Return  1  if  the (?J) or (?-J) option setting is used in the pattern,
       otherwise 0. The fourth argument should point to an int variable.  (?J)
       and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.

         PCRE_INFO_JIT

       Return  1  if  the pattern was studied with one of the JIT options, and
       just-in-time compiling was successful. The fourth argument should point
       to  an  int variable. A return value of 0 means that JIT support is not
       available in this version of PCRE, or that the pattern was not  studied
       with  a JIT option, or that the JIT compiler could not handle this par-
       ticular pattern. See the pcrejit documentation for details of what  can
       and cannot be handled.

         PCRE_INFO_JITSIZE

       If  the  pattern was successfully studied with a JIT option, return the
       size of the JIT compiled code, otherwise return zero. The fourth  argu-
       ment should point to a size_t variable.

         PCRE_INFO_LASTLITERAL

       Return  the value of the rightmost literal data unit that must exist in
       any matched string, other than at its start, if such a value  has  been
       recorded. The fourth argument should point to an int variable. If there
       is no such value, -1 is returned. For anchored patterns, a last literal
       value  is recorded only if it follows something of variable length. For
       example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
       /^a\dz\d/ the returned value is -1.

         PCRE_INFO_MAXLOOKBEHIND

       Return  the  number of characters (NB not bytes) in the longest lookbe-
       hind assertion in the pattern. Note that the simple assertions  \b  and
       \B  require a one-character lookbehind. This information is useful when
       doing multi-segment matching using the partial matching facilities.

         PCRE_INFO_MINLENGTH

       If the pattern was studied and a minimum length  for  matching  subject
       strings  was  computed,  its  value is returned. Otherwise the returned
       value is -1. The value is a number of characters, which in  UTF-8  mode
       may  be  different from the number of bytes. The fourth argument should
       point to an int variable. A non-negative value is a lower bound to  the
       length  of  any  matching  string. There may not be any strings of that
       length that do actually match, but every string that does match  is  at
       least that long.

         PCRE_INFO_NAMECOUNT
         PCRE_INFO_NAMEENTRYSIZE
         PCRE_INFO_NAMETABLE

       PCRE  supports the use of named as well as numbered capturing parenthe-
       ses. The names are just an additional way of identifying the  parenthe-
       ses, which still acquire numbers. Several convenience functions such as
       pcre_get_named_substring() are provided for  extracting  captured  sub-
       strings  by  name. It is also possible to extract the data directly, by
       first converting the name to a number in order to  access  the  correct
       pointers in the output vector (described with pcre_exec() below). To do
       the conversion, you need  to  use  the  name-to-number  map,  which  is
       described by these three values.

       The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
       gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
       of  each  entry;  both  of  these  return  an int value. The entry size
       depends on the length of the longest name. PCRE_INFO_NAMETABLE  returns
       a pointer to the first entry of the table. This is a pointer to char in
       the 8-bit library, where the first two bytes of each entry are the num-
       ber  of  the capturing parenthesis, most significant byte first. In the
       16-bit library, the pointer points to 16-bit data units, the  first  of
       which  contains  the  parenthesis  number. The rest of the entry is the
       corresponding name, zero terminated.

       The names are in alphabetical order. Duplicate names may appear if  (?|
       is used to create multiple groups with the same number, as described in
       the section on duplicate subpattern numbers in  the  pcrepattern  page.
       Duplicate  names  for  subpatterns with different numbers are permitted
       only if PCRE_DUPNAMES is set. In all cases  of  duplicate  names,  they
       appear  in  the table in the order in which they were found in the pat-
       tern. In the absence of (?| this is the  order  of  increasing  number;
       when (?| is used this is not necessarily the case because later subpat-
       terns may have lower numbers.

       As a simple example of the name/number table,  consider  the  following
       pattern after compilation by the 8-bit library (assume PCRE_EXTENDED is
       set, so white space - including newlines - is ignored):

         (?<date> (?<year>(\d\d)?\d\d) -
         (?<month>\d\d) - (?<day>\d\d) )

       There are four named subpatterns, so the table has  four  entries,  and
       each  entry  in the table is eight bytes long. The table is as follows,
       with non-printing bytes shows in hexadecimal, and undefined bytes shown
       as ??:

         00 01 d  a  t  e  00 ??
         00 05 d  a  y  00 ?? ??
         00 04 m  o  n  t  h  00
         00 02 y  e  a  r  00 ??

       When  writing  code  to  extract  data from named subpatterns using the
       name-to-number map, remember that the length of the entries  is  likely
       to be different for each compiled pattern.

         PCRE_INFO_OKPARTIAL

       Return  1  if  the  pattern  can  be  used  for  partial  matching with
       pcre_exec(), otherwise 0. The fourth argument should point  to  an  int
       variable.  From  release  8.00,  this  always  returns  1,  because the
       restrictions that previously applied  to  partial  matching  have  been
       lifted.  The  pcrepartial documentation gives details of partial match-
       ing.

         PCRE_INFO_OPTIONS

       Return a copy of the options with which the pattern was  compiled.  The
       fourth  argument  should  point to an unsigned long int variable. These
       option bits are those specified in the call to pcre_compile(), modified
       by any top-level option settings at the start of the pattern itself. In
       other words, they are the options that will be in force  when  matching
       starts.  For  example, if the pattern /(?im)abc(?-i)d/ is compiled with
       the PCRE_EXTENDED option, the result is PCRE_CASELESS,  PCRE_MULTILINE,
       and PCRE_EXTENDED.

       A  pattern  is  automatically  anchored by PCRE if all of its top-level
       alternatives begin with one of the following:

         ^     unless PCRE_MULTILINE is set
         \A    always
         \G    always
         .*    if PCRE_DOTALL is set and there are no back
                 references to the subpattern in which .* appears

       For such patterns, the PCRE_ANCHORED bit is set in the options returned
       by pcre_fullinfo().

         PCRE_INFO_SIZE

       Return  the size of the compiled pattern in bytes (for both libraries).
       The fourth argument should point to a size_t variable. This value  does
       not  include  the  size  of  the  pcre  structure  that  is returned by
       pcre_compile(). The value that is passed as the argument  to  pcre_mal-
       loc()  when pcre_compile() is getting memory in which to place the com-
       piled data is the value returned by this option plus the  size  of  the
       pcre  structure. Studying a compiled pattern, with or without JIT, does
       not alter the value returned by this option.

         PCRE_INFO_STUDYSIZE

       Return the size in bytes of the data block pointed to by the study_data
       field  in  a  pcre_extra  block.  If pcre_extra is NULL, or there is no
       study data, zero is returned. The fourth argument  should  point  to  a
       size_t  variable. The study_data field is set by pcre_study() to record
       information that will speed  up  matching  (see  the  section  entitled
       "Studying a pattern" above). The format of the study_data block is pri-
       vate, but its length is made available via this option so that  it  can
       be  saved  and  restored  (see  the  pcreprecompile  documentation  for
       details).


REFERENCE COUNTS

       int pcre_refcount(pcre *code, int adjust);

       The pcre_refcount() function is used to maintain a reference  count  in
       the data block that contains a compiled pattern. It is provided for the
       benefit of applications that  operate  in  an  object-oriented  manner,
       where different parts of the application may be using the same compiled
       pattern, but you want to free the block when they are all done.

       When a pattern is compiled, the reference count field is initialized to
       zero.   It is changed only by calling this function, whose action is to
       add the adjust value (which may be positive or  negative)  to  it.  The
       yield of the function is the new value. However, the value of the count
       is constrained to lie between 0 and 65535, inclusive. If the new  value
       is outside these limits, it is forced to the appropriate limit value.

       Except  when it is zero, the reference count is not correctly preserved
       if a pattern is compiled on one host and then  transferred  to  a  host
       whose byte-order is different. (This seems a highly unlikely scenario.)


MATCHING A PATTERN: THE TRADITIONAL FUNCTION

       int pcre_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       The  function pcre_exec() is called to match a subject string against a
       compiled pattern, which is passed in the code argument. If the  pattern
       was  studied,  the  result  of  the study should be passed in the extra
       argument. You can call pcre_exec() with the same code and  extra  argu-
       ments  as  many  times as you like, in order to match different subject
       strings with the same pattern.

       This function is the main matching facility  of  the  library,  and  it
       operates  in  a  Perl-like  manner. For specialist use there is also an
       alternative matching function, which is described below in the  section
       about the pcre_dfa_exec() function.

       In  most applications, the pattern will have been compiled (and option-
       ally studied) in the same process that calls pcre_exec().  However,  it
       is possible to save compiled patterns and study data, and then use them
       later in different processes, possibly even on different hosts.  For  a
       discussion about this, see the pcreprecompile documentation.

       Here is an example of a simple call to pcre_exec():

         int rc;
         int ovector[30];
         rc = pcre_exec(
           re,             /* result of pcre_compile() */
           NULL,           /* we didn't study the pattern */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           ovector,        /* vector of integers for substring information */
           30);            /* number of elements (NOT size in bytes) */

   Extra data for pcre_exec()

       If  the  extra argument is not NULL, it must point to a pcre_extra data
       block. The pcre_study() function returns such a block (when it  doesn't
       return  NULL), but you can also create one for yourself, and pass addi-
       tional information in it. The pcre_extra block contains  the  following
       fields (not necessarily in this order):

         unsigned long int flags;
         void *study_data;
         void *executable_jit;
         unsigned long int match_limit;
         unsigned long int match_limit_recursion;
         void *callout_data;
         const unsigned char *tables;
         unsigned char **mark;

       In  the  16-bit  version  of  this  structure,  the mark field has type
       "PCRE_UCHAR16 **".

       The flags field is used to specify which of the other fields  are  set.
       The flag bits are:

         PCRE_EXTRA_CALLOUT_DATA
         PCRE_EXTRA_EXECUTABLE_JIT
         PCRE_EXTRA_MARK
         PCRE_EXTRA_MATCH_LIMIT
         PCRE_EXTRA_MATCH_LIMIT_RECURSION
         PCRE_EXTRA_STUDY_DATA
         PCRE_EXTRA_TABLES

       Other  flag  bits should be set to zero. The study_data field and some-
       times the executable_jit field are set in the pcre_extra block that  is
       returned  by pcre_study(), together with the appropriate flag bits. You
       should not set these yourself, but you may add to the block by  setting
       other fields and their corresponding flag bits.

       The match_limit field provides a means of preventing PCRE from using up
       a vast amount of resources when running patterns that are not going  to
       match,  but  which  have  a very large number of possibilities in their
       search trees. The classic example is a pattern that uses nested  unlim-
       ited repeats.

       Internally,  pcre_exec() uses a function called match(), which it calls
       repeatedly (sometimes recursively). The limit  set  by  match_limit  is
       imposed  on the number of times this function is called during a match,
       which has the effect of limiting the amount of  backtracking  that  can
       take place. For patterns that are not anchored, the count restarts from
       zero for each position in the subject string.

       When pcre_exec() is called with a pattern that was successfully studied
       with  a  JIT  option, the way that the matching is executed is entirely
       different.  However, there is still the possibility of runaway matching
       that goes on for a very long time, and so the match_limit value is also
       used in this case (but in a different way) to limit how long the match-
       ing can continue.

       The  default  value  for  the  limit can be set when PCRE is built; the
       default default is 10 million, which handles all but the  most  extreme
       cases.  You  can  override  the  default by suppling pcre_exec() with a
       pcre_extra    block    in    which    match_limit    is    set,     and
       PCRE_EXTRA_MATCH_LIMIT  is  set  in  the  flags  field. If the limit is
       exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.

       The match_limit_recursion field is similar to match_limit, but  instead
       of limiting the total number of times that match() is called, it limits
       the depth of recursion. The recursion depth is a  smaller  number  than
       the  total number of calls, because not all calls to match() are recur-
       sive.  This limit is of use only if it is set smaller than match_limit.

       Limiting the recursion depth limits the amount of  machine  stack  that
       can  be used, or, when PCRE has been compiled to use memory on the heap
       instead of the stack, the amount of heap memory that can be used.  This
       limit  is not relevant, and is ignored, when matching is done using JIT
       compiled code.

       The default value for match_limit_recursion can be  set  when  PCRE  is
       built;  the  default  default  is  the  same  value  as the default for
       match_limit. You can override the default by suppling pcre_exec()  with
       a   pcre_extra   block  in  which  match_limit_recursion  is  set,  and
       PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in  the  flags  field.  If  the
       limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.

       The  callout_data  field is used in conjunction with the "callout" fea-
       ture, and is described in the pcrecallout documentation.

       The tables field  is  used  to  pass  a  character  tables  pointer  to
       pcre_exec();  this overrides the value that is stored with the compiled
       pattern. A non-NULL value is stored with the compiled pattern  only  if
       custom  tables  were  supplied to pcre_compile() via its tableptr argu-
       ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
       PCRE's  internal  tables  to be used. This facility is helpful when re-
       using patterns that have been saved after compiling  with  an  external
       set  of  tables,  because  the  external tables might be at a different
       address when pcre_exec() is called. See the  pcreprecompile  documenta-
       tion for a discussion of saving compiled patterns for later use.

       If  PCRE_EXTRA_MARK  is  set in the flags field, the mark field must be
       set to point to a suitable variable. If the pattern contains any  back-
       tracking  control verbs such as (*MARK:NAME), and the execution ends up
       with a name to pass back, a pointer to the  name  string  (zero  termi-
       nated)  is  placed  in  the  variable pointed to by the mark field. The
       names are within the compiled pattern; if you wish  to  retain  such  a
       name  you must copy it before freeing the memory of a compiled pattern.
       If there is no name to pass back, the variable pointed to by  the  mark
       field  is  set  to NULL. For details of the backtracking control verbs,
       see the section entitled "Backtracking control" in the pcrepattern doc-
       umentation.

   Option bits for pcre_exec()

       The  unused  bits of the options argument for pcre_exec() must be zero.
       The only bits that may  be  set  are  PCRE_ANCHORED,  PCRE_NEWLINE_xxx,
       PCRE_NOTBOL,    PCRE_NOTEOL,    PCRE_NOTEMPTY,   PCRE_NOTEMPTY_ATSTART,
       PCRE_NO_START_OPTIMIZE,  PCRE_NO_UTF8_CHECK,   PCRE_PARTIAL_HARD,   and
       PCRE_PARTIAL_SOFT.

       If  the  pattern  was successfully studied with one of the just-in-time
       (JIT) compile options, the only supported options for JIT execution are
       PCRE_NO_UTF8_CHECK,     PCRE_NOTBOL,     PCRE_NOTEOL,    PCRE_NOTEMPTY,
       PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT. If  an
       unsupported  option  is  used, JIT execution is disabled and the normal
       interpretive code in pcre_exec() is run.

         PCRE_ANCHORED

       The PCRE_ANCHORED option limits pcre_exec() to matching  at  the  first
       matching  position.  If  a  pattern was compiled with PCRE_ANCHORED, or
       turned out to be anchored by virtue of its contents, it cannot be  made
       unachored at matching time.

         PCRE_BSR_ANYCRLF
         PCRE_BSR_UNICODE

       These options (which are mutually exclusive) control what the \R escape
       sequence matches. The choice is either to match only CR, LF,  or  CRLF,
       or  to  match  any Unicode newline sequence. These options override the
       choice that was made or defaulted when the pattern was compiled.

         PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

       These options override  the  newline  definition  that  was  chosen  or
       defaulted  when the pattern was compiled. For details, see the descrip-
       tion of pcre_compile()  above.  During  matching,  the  newline  choice
       affects  the  behaviour  of the dot, circumflex, and dollar metacharac-
       ters. It may also alter the way the match position is advanced after  a
       match failure for an unanchored pattern.

       When  PCRE_NEWLINE_CRLF,  PCRE_NEWLINE_ANYCRLF,  or PCRE_NEWLINE_ANY is
       set, and a match attempt for an unanchored pattern fails when the  cur-
       rent  position  is  at  a  CRLF  sequence,  and the pattern contains no
       explicit matches for  CR  or  LF  characters,  the  match  position  is
       advanced by two characters instead of one, in other words, to after the
       CRLF.

       The above rule is a compromise that makes the most common cases work as
       expected.  For  example,  if  the  pattern  is .+A (and the PCRE_DOTALL
       option is not set), it does not match the string "\r\nA" because, after
       failing  at the start, it skips both the CR and the LF before retrying.
       However, the pattern [\r\n]A does match that string,  because  it  con-
       tains an explicit CR or LF reference, and so advances only by one char-
       acter after the first failure.

       An explicit match for CR of LF is either a literal appearance of one of
       those  characters,  or  one  of the \r or \n escape sequences. Implicit
       matches such as [^X] do not count, nor does \s (which includes  CR  and
       LF in the characters that it matches).

       Notwithstanding  the above, anomalous effects may still occur when CRLF
       is a valid newline sequence and explicit \r or \n escapes appear in the
       pattern.

         PCRE_NOTBOL

       This option specifies that first character of the subject string is not
       the beginning of a line, so the  circumflex  metacharacter  should  not
       match  before it. Setting this without PCRE_MULTILINE (at compile time)
       causes circumflex never to match. This option affects only  the  behav-
       iour of the circumflex metacharacter. It does not affect \A.

         PCRE_NOTEOL

       This option specifies that the end of the subject string is not the end
       of a line, so the dollar metacharacter should not match it nor  (except
       in  multiline mode) a newline immediately before it. Setting this with-
       out PCRE_MULTILINE (at compile time) causes dollar never to match. This
       option  affects only the behaviour of the dollar metacharacter. It does
       not affect \Z or \z.

         PCRE_NOTEMPTY

       An empty string is not considered to be a valid match if this option is
       set.  If  there are alternatives in the pattern, they are tried. If all
       the alternatives match the empty string, the entire  match  fails.  For
       example, if the pattern

         a?b?

       is  applied  to  a  string not beginning with "a" or "b", it matches an
       empty string at the start of the subject. With PCRE_NOTEMPTY set,  this
       match is not valid, so PCRE searches further into the string for occur-
       rences of "a" or "b".

         PCRE_NOTEMPTY_ATSTART

       This is like PCRE_NOTEMPTY, except that an empty string match  that  is
       not  at  the  start  of  the  subject  is  permitted. If the pattern is
       anchored, such a match can occur only if the pattern contains \K.

       Perl    has    no    direct    equivalent    of    PCRE_NOTEMPTY     or
       PCRE_NOTEMPTY_ATSTART,  but  it  does  make a special case of a pattern
       match of the empty string within its split() function, and  when  using
       the  /g  modifier.  It  is  possible  to emulate Perl's behaviour after
       matching a null string by first trying the match again at the same off-
       set  with  PCRE_NOTEMPTY_ATSTART  and  PCRE_ANCHORED,  and then if that
       fails, by advancing the starting offset (see below) and trying an ordi-
       nary  match  again. There is some code that demonstrates how to do this
       in the pcredemo sample program. In the most general case, you  have  to
       check  to  see  if the newline convention recognizes CRLF as a newline,
       and if so, and the current character is CR followed by LF, advance  the
       starting offset by two characters instead of one.

         PCRE_NO_START_OPTIMIZE

       There  are a number of optimizations that pcre_exec() uses at the start
       of a match, in order to speed up the process. For  example,  if  it  is
       known that an unanchored match must start with a specific character, it
       searches the subject for that character, and fails  immediately  if  it
       cannot  find  it,  without actually running the main matching function.
       This means that a special item such as (*COMMIT) at the start of a pat-
       tern  is  not  considered until after a suitable starting point for the
       match has been found. When callouts or (*MARK) items are in use,  these
       "start-up" optimizations can cause them to be skipped if the pattern is
       never actually used. The start-up optimizations are in  effect  a  pre-
       scan of the subject that takes place before the pattern is run.

       The  PCRE_NO_START_OPTIMIZE option disables the start-up optimizations,
       possibly causing performance to suffer,  but  ensuring  that  in  cases
       where  the  result is "no match", the callouts do occur, and that items
       such as (*COMMIT) and (*MARK) are considered at every possible starting
       position  in  the  subject  string. If PCRE_NO_START_OPTIMIZE is set at
       compile time,  it  cannot  be  unset  at  matching  time.  The  use  of
       PCRE_NO_START_OPTIMIZE disables JIT execution; when it is set, matching
       is always done using interpretively.

       Setting PCRE_NO_START_OPTIMIZE can change the  outcome  of  a  matching
       operation.  Consider the pattern

         (*COMMIT)ABC

       When  this  is  compiled, PCRE records the fact that a match must start
       with the character "A". Suppose the subject  string  is  "DEFABC".  The
       start-up  optimization  scans along the subject, finds "A" and runs the
       first match attempt from there. The (*COMMIT) item means that the  pat-
       tern  must  match the current starting position, which in this case, it
       does. However, if the same match  is  run  with  PCRE_NO_START_OPTIMIZE
       set,  the  initial  scan  along the subject string does not happen. The
       first match attempt is run starting  from  "D"  and  when  this  fails,
       (*COMMIT)  prevents  any  further  matches  being tried, so the overall
       result is "no match". If the pattern is studied,  more  start-up  opti-
       mizations  may  be  used. For example, a minimum length for the subject
       may be recorded. Consider the pattern

         (*MARK:A)(X|Y)

       The minimum length for a match is one  character.  If  the  subject  is
       "ABC",  there  will  be  attempts  to  match "ABC", "BC", "C", and then
       finally an empty string.  If the pattern is studied, the final  attempt
       does  not take place, because PCRE knows that the subject is too short,
       and so the (*MARK) is never encountered.  In this  case,  studying  the
       pattern  does  not  affect the overall match result, which is still "no
       match", but it does affect the auxiliary information that is returned.

         PCRE_NO_UTF8_CHECK

       When PCRE_UTF8 is set at compile time, the validity of the subject as a
       UTF-8  string is automatically checked when pcre_exec() is subsequently
       called.  The entire string is checked before any other processing takes
       place.  The  value  of  startoffset  is  also checked to ensure that it
       points to the start of a UTF-8 character. There is a  discussion  about
       the  validity  of  UTF-8 strings in the pcreunicode page. If an invalid
       sequence  of  bytes   is   found,   pcre_exec()   returns   the   error
       PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and the problem is a
       truncated character at the end of the subject, PCRE_ERROR_SHORTUTF8. In
       both  cases, information about the precise nature of the error may also
       be returned (see the descriptions of these errors in the section  enti-
       tled  Error return values from pcre_exec() below).  If startoffset con-
       tains a value that does not point to the start of a UTF-8 character (or
       to the end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned.

       If  you  already  know that your subject is valid, and you want to skip
       these   checks   for   performance   reasons,   you   can    set    the
       PCRE_NO_UTF8_CHECK  option  when calling pcre_exec(). You might want to
       do this for the second and subsequent calls to pcre_exec() if  you  are
       making  repeated  calls  to  find  all  the matches in a single subject
       string. However, you should be  sure  that  the  value  of  startoffset
       points  to  the  start of a character (or the end of the subject). When
       PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid string as a
       subject  or  an invalid value of startoffset is undefined. Your program
       may crash.

         PCRE_PARTIAL_HARD
         PCRE_PARTIAL_SOFT

       These options turn on the partial matching feature. For backwards  com-
       patibility,  PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial
       match occurs if the end of the subject string is reached  successfully,
       but  there  are not enough subject characters to complete the match. If
       this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set,
       matching  continues  by  testing any remaining alternatives. Only if no
       complete match can be found is PCRE_ERROR_PARTIAL returned  instead  of
       PCRE_ERROR_NOMATCH.  In  other  words,  PCRE_PARTIAL_SOFT says that the
       caller is prepared to handle a partial match, but only if  no  complete
       match can be found.

       If  PCRE_PARTIAL_HARD  is  set, it overrides PCRE_PARTIAL_SOFT. In this
       case, if a partial match  is  found,  pcre_exec()  immediately  returns
       PCRE_ERROR_PARTIAL,  without  considering  any  other  alternatives. In
       other words, when PCRE_PARTIAL_HARD is set, a partial match is  consid-
       ered to be more important that an alternative complete match.

       In  both  cases,  the portion of the string that was inspected when the
       partial match was found is set as the first matching string. There is a
       more  detailed  discussion  of partial and multi-segment matching, with
       examples, in the pcrepartial documentation.

   The string to be matched by pcre_exec()

       The subject string is passed to pcre_exec() as a pointer in subject,  a
       length  in  bytes in length, and a starting byte offset in startoffset.
       If this is  negative  or  greater  than  the  length  of  the  subject,
       pcre_exec()  returns  PCRE_ERROR_BADOFFSET. When the starting offset is
       zero, the search for a match starts at the beginning  of  the  subject,
       and this is by far the most common case. In UTF-8 mode, the byte offset
       must point to the start of a UTF-8 character (or the end  of  the  sub-
       ject).  Unlike  the pattern string, the subject may contain binary zero
       bytes.

       A non-zero starting offset is useful when searching for  another  match
       in  the same subject by calling pcre_exec() again after a previous suc-
       cess.  Setting startoffset differs from just passing over  a  shortened
       string  and  setting  PCRE_NOTBOL  in the case of a pattern that begins
       with any kind of lookbehind. For example, consider the pattern

         \Biss\B

       which finds occurrences of "iss" in the middle of  words.  (\B  matches
       only  if  the  current position in the subject is not a word boundary.)
       When applied to the string "Mississipi" the first call  to  pcre_exec()
       finds  the  first  occurrence. If pcre_exec() is called again with just
       the remainder of the subject,  namely  "issipi",  it  does  not  match,
       because \B is always false at the start of the subject, which is deemed
       to be a word boundary. However, if pcre_exec()  is  passed  the  entire
       string again, but with startoffset set to 4, it finds the second occur-
       rence of "iss" because it is able to look behind the starting point  to
       discover that it is preceded by a letter.

       Finding  all  the  matches  in a subject is tricky when the pattern can
       match an empty string. It is possible to emulate Perl's /g behaviour by
       first   trying   the   match   again  at  the  same  offset,  with  the
       PCRE_NOTEMPTY_ATSTART and  PCRE_ANCHORED  options,  and  then  if  that
       fails,  advancing  the  starting  offset  and  trying an ordinary match
       again. There is some code that demonstrates how to do this in the pcre-
       demo sample program. In the most general case, you have to check to see
       if the newline convention recognizes CRLF as a newline, and if so,  and
       the current character is CR followed by LF, advance the starting offset
       by two characters instead of one.

       If a non-zero starting offset is passed when the pattern  is  anchored,
       one attempt to match at the given offset is made. This can only succeed
       if the pattern does not require the match to be at  the  start  of  the
       subject.

   How pcre_exec() returns captured substrings

       In  general, a pattern matches a certain portion of the subject, and in
       addition, further substrings from the subject  may  be  picked  out  by
       parts  of  the  pattern.  Following the usage in Jeffrey Friedl's book,
       this is called "capturing" in what follows, and the  phrase  "capturing
       subpattern"  is  used for a fragment of a pattern that picks out a sub-
       string. PCRE supports several other kinds of  parenthesized  subpattern
       that do not cause substrings to be captured.

       Captured substrings are returned to the caller via a vector of integers
       whose address is passed in ovector. The number of elements in the  vec-
       tor  is  passed in ovecsize, which must be a non-negative number. Note:
       this argument is NOT the size of ovector in bytes.

       The first two-thirds of the vector is used to pass back  captured  sub-
       strings,  each  substring using a pair of integers. The remaining third
       of the vector is used as workspace by pcre_exec() while  matching  cap-
       turing  subpatterns, and is not available for passing back information.
       The number passed in ovecsize should always be a multiple of three.  If
       it is not, it is rounded down.

       When  a  match  is successful, information about captured substrings is
       returned in pairs of integers, starting at the  beginning  of  ovector,
       and  continuing  up  to two-thirds of its length at the most. The first
       element of each pair is set to the byte offset of the  first  character
       in  a  substring, and the second is set to the byte offset of the first
       character after the end of a substring. Note: these values  are  always
       byte offsets, even in UTF-8 mode. They are not character counts.

       The  first  pair  of  integers, ovector[0] and ovector[1], identify the
       portion of the subject string matched by the entire pattern.  The  next
       pair  is  used for the first capturing subpattern, and so on. The value
       returned by pcre_exec() is one more than the highest numbered pair that
       has  been  set.  For example, if two substrings have been captured, the
       returned value is 3. If there are no capturing subpatterns, the  return
       value from a successful match is 1, indicating that just the first pair
       of offsets has been set.

       If a capturing subpattern is matched repeatedly, it is the last portion
       of the string that it matched that is returned.

       If  the vector is too small to hold all the captured substring offsets,
       it is used as far as possible (up to two-thirds of its length), and the
       function  returns a value of zero. If neither the actual string matched
       nor any captured substrings are of interest, pcre_exec() may be  called
       with  ovector passed as NULL and ovecsize as zero. However, if the pat-
       tern contains back references and the ovector  is  not  big  enough  to
       remember  the related substrings, PCRE has to get additional memory for
       use during matching. Thus it is usually advisable to supply an  ovector
       of reasonable size.

       There  are  some  cases where zero is returned (indicating vector over-
       flow) when in fact the vector is exactly the right size for  the  final
       match. For example, consider the pattern

         (a)(?:(b)c|bd)

       If  a  vector of 6 elements (allowing for only 1 captured substring) is
       given with subject string "abd", pcre_exec() will try to set the second
       captured string, thereby recording a vector overflow, before failing to
       match "c" and backing up  to  try  the  second  alternative.  The  zero
       return,  however,  does  correctly  indicate that the maximum number of
       slots (namely 2) have been filled. In similar cases where there is tem-
       porary  overflow,  but  the final number of used slots is actually less
       than the maximum, a non-zero value is returned.

       The pcre_fullinfo() function can be used to find out how many capturing
       subpatterns  there  are  in  a  compiled pattern. The smallest size for
       ovector that will allow for n captured substrings, in addition  to  the
       offsets of the substring matched by the whole pattern, is (n+1)*3.

       It  is  possible for capturing subpattern number n+1 to match some part
       of the subject when subpattern n has not been used at all. For example,
       if  the  string  "abc"  is  matched against the pattern (a|(z))(bc) the
       return from the function is 4, and subpatterns 1 and 3 are matched, but
       2  is  not.  When  this happens, both values in the offset pairs corre-
       sponding to unused subpatterns are set to -1.

       Offset values that correspond to unused subpatterns at the end  of  the
       expression  are  also  set  to  -1. For example, if the string "abc" is
       matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are  not
       matched.  The  return  from the function is 2, because the highest used
       capturing subpattern number is 1, and the offsets for  for  the  second
       and  third  capturing subpatterns (assuming the vector is large enough,
       of course) are set to -1.

       Note: Elements in the first two-thirds of ovector that  do  not  corre-
       spond  to  capturing parentheses in the pattern are never changed. That
       is, if a pattern contains n capturing parentheses, no more  than  ovec-
       tor[0]  to ovector[2n+1] are set by pcre_exec(). The other elements (in
       the first two-thirds) retain whatever values they previously had.

       Some convenience functions are provided  for  extracting  the  captured
       substrings as separate strings. These are described below.

   Error return values from pcre_exec()

       If  pcre_exec()  fails, it returns a negative number. The following are
       defined in the header file:

         PCRE_ERROR_NOMATCH        (-1)

       The subject string did not match the pattern.

         PCRE_ERROR_NULL           (-2)

       Either code or subject was passed as NULL,  or  ovector  was  NULL  and
       ovecsize was not zero.

         PCRE_ERROR_BADOPTION      (-3)

       An unrecognized bit was set in the options argument.

         PCRE_ERROR_BADMAGIC       (-4)

       PCRE  stores a 4-byte "magic number" at the start of the compiled code,
       to catch the case when it is passed a junk pointer and to detect when a
       pattern that was compiled in an environment of one endianness is run in
       an environment with the other endianness. This is the error  that  PCRE
       gives when the magic number is not present.

         PCRE_ERROR_UNKNOWN_OPCODE (-5)

       While running the pattern match, an unknown item was encountered in the
       compiled pattern. This error could be caused by a bug  in  PCRE  or  by
       overwriting of the compiled pattern.

         PCRE_ERROR_NOMEMORY       (-6)

       If  a  pattern contains back references, but the ovector that is passed
       to pcre_exec() is not big enough to remember the referenced substrings,
       PCRE  gets  a  block of memory at the start of matching to use for this
       purpose. If the call via pcre_malloc() fails, this error is given.  The
       memory is automatically freed at the end of matching.

       This  error  is also given if pcre_stack_malloc() fails in pcre_exec().
       This can happen only when PCRE has been compiled with  --disable-stack-
       for-recursion.

         PCRE_ERROR_NOSUBSTRING    (-7)

       This  error is used by the pcre_copy_substring(), pcre_get_substring(),
       and  pcre_get_substring_list()  functions  (see  below).  It  is  never
       returned by pcre_exec().

         PCRE_ERROR_MATCHLIMIT     (-8)

       The  backtracking  limit,  as  specified  by the match_limit field in a
       pcre_extra structure (or defaulted) was reached.  See  the  description
       above.

         PCRE_ERROR_CALLOUT        (-9)

       This error is never generated by pcre_exec() itself. It is provided for
       use by callout functions that want to yield a distinctive  error  code.
       See the pcrecallout documentation for details.

         PCRE_ERROR_BADUTF8        (-10)

       A  string  that contains an invalid UTF-8 byte sequence was passed as a
       subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size  of
       the  output  vector  (ovecsize)  is  at least 2, the byte offset to the
       start of the the invalid UTF-8 character is placed in  the  first  ele-
       ment,  and  a  reason  code is placed in the second element. The reason
       codes are listed in the following section.  For backward compatibility,
       if  PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char-
       acter  at  the  end  of  the   subject   (reason   codes   1   to   5),
       PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8.

         PCRE_ERROR_BADUTF8_OFFSET (-11)

       The  UTF-8  byte  sequence that was passed as a subject was checked and
       found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but  the
       value  of startoffset did not point to the beginning of a UTF-8 charac-
       ter or the end of the subject.

         PCRE_ERROR_PARTIAL        (-12)

       The subject string did not match, but it did match partially.  See  the
       pcrepartial documentation for details of partial matching.

         PCRE_ERROR_BADPARTIAL     (-13)

       This  code  is  no  longer  in  use.  It was formerly returned when the
       PCRE_PARTIAL option was used with a compiled pattern  containing  items
       that  were  not  supported  for  partial  matching.  From  release 8.00
       onwards, there are no restrictions on partial matching.

         PCRE_ERROR_INTERNAL       (-14)

       An unexpected internal error has occurred. This error could  be  caused
       by a bug in PCRE or by overwriting of the compiled pattern.

         PCRE_ERROR_BADCOUNT       (-15)

       This error is given if the value of the ovecsize argument is negative.

         PCRE_ERROR_RECURSIONLIMIT (-21)

       The internal recursion limit, as specified by the match_limit_recursion
       field in a pcre_extra structure (or defaulted)  was  reached.  See  the
       description above.

         PCRE_ERROR_BADNEWLINE     (-23)

       An invalid combination of PCRE_NEWLINE_xxx options was given.

         PCRE_ERROR_BADOFFSET      (-24)

       The value of startoffset was negative or greater than the length of the
       subject, that is, the value in length.

         PCRE_ERROR_SHORTUTF8      (-25)

       This error is returned instead of PCRE_ERROR_BADUTF8 when  the  subject
       string  ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD
       option is set.  Information  about  the  failure  is  returned  as  for
       PCRE_ERROR_BADUTF8.  It  is in fact sufficient to detect this case, but
       this special error code for PCRE_PARTIAL_HARD precedes the  implementa-
       tion  of returned information; it is retained for backwards compatibil-
       ity.

         PCRE_ERROR_RECURSELOOP    (-26)

       This error is returned when pcre_exec() detects a recursion loop within
       the  pattern. Specifically, it means that either the whole pattern or a
       subpattern has been called recursively for the second time at the  same
       position in the subject string. Some simple patterns that might do this
       are detected and faulted at compile time, but more  complicated  cases,
       in particular mutual recursions between two different subpatterns, can-
       not be detected until run time.

         PCRE_ERROR_JIT_STACKLIMIT (-27)

       This error is returned when a pattern  that  was  successfully  studied
       using  a  JIT compile option is being matched, but the memory available
       for the just-in-time processing stack is  not  large  enough.  See  the
       pcrejit documentation for more details.

         PCRE_ERROR_BADMODE        (-28)

       This error is given if a pattern that was compiled by the 8-bit library
       is passed to a 16-bit library function, or vice versa.

         PCRE_ERROR_BADENDIANNESS  (-29)

       This error is given if  a  pattern  that  was  compiled  and  saved  is
       reloaded  on  a  host  with  different endianness. The utility function
       pcre_pattern_to_host_byte_order() can be used to convert such a pattern
       so that it runs on the new host.

       Error numbers -16 to -20, -22, and -30 are not used by pcre_exec().

   Reason codes for invalid UTF-8 strings

       This  section  applies  only  to  the  8-bit library. The corresponding
       information for the 16-bit library is given in the pcre16 page.

       When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT-
       UTF8,  and  the size of the output vector (ovecsize) is at least 2, the
       offset of the start of the invalid UTF-8 character  is  placed  in  the
       first output vector element (ovector[0]) and a reason code is placed in
       the second element (ovector[1]). The reason codes are  given  names  in
       the pcre.h header file:

         PCRE_UTF8_ERR1
         PCRE_UTF8_ERR2
         PCRE_UTF8_ERR3
         PCRE_UTF8_ERR4
         PCRE_UTF8_ERR5

       The  string  ends  with a truncated UTF-8 character; the code specifies
       how many bytes are missing (1 to 5). Although RFC 3629 restricts  UTF-8
       characters  to  be  no longer than 4 bytes, the encoding scheme (origi-
       nally defined by RFC 2279) allows for  up  to  6  bytes,  and  this  is
       checked first; hence the possibility of 4 or 5 missing bytes.

         PCRE_UTF8_ERR6
         PCRE_UTF8_ERR7
         PCRE_UTF8_ERR8
         PCRE_UTF8_ERR9
         PCRE_UTF8_ERR10

       The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
       the character do not have the binary value 0b10 (that  is,  either  the
       most significant bit is 0, or the next bit is 1).

         PCRE_UTF8_ERR11
         PCRE_UTF8_ERR12

       A  character that is valid by the RFC 2279 rules is either 5 or 6 bytes
       long; these code points are excluded by RFC 3629.

         PCRE_UTF8_ERR13

       A 4-byte character has a value greater than 0x10fff; these code  points
       are excluded by RFC 3629.

         PCRE_UTF8_ERR14

       A  3-byte  character  has  a  value in the range 0xd800 to 0xdfff; this
       range of code points are reserved by RFC 3629 for use with UTF-16,  and
       so are excluded from UTF-8.

         PCRE_UTF8_ERR15
         PCRE_UTF8_ERR16
         PCRE_UTF8_ERR17
         PCRE_UTF8_ERR18
         PCRE_UTF8_ERR19

       A  2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes
       for a value that can be represented by fewer bytes, which  is  invalid.
       For  example,  the two bytes 0xc0, 0xae give the value 0x2e, whose cor-
       rect coding uses just one byte.

         PCRE_UTF8_ERR20

       The two most significant bits of the first byte of a character have the
       binary  value 0b10 (that is, the most significant bit is 1 and the sec-
       ond is 0). Such a byte can only validly occur as the second  or  subse-
       quent byte of a multi-byte character.

         PCRE_UTF8_ERR21

       The  first byte of a character has the value 0xfe or 0xff. These values
       can never occur in a valid UTF-8 string.


EXTRACTING CAPTURED SUBSTRINGS BY NUMBER

       int pcre_copy_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber, char *buffer,
            int buffersize);

       int pcre_get_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber,
            const char **stringptr);

       int pcre_get_substring_list(const char *subject,
            int *ovector, int stringcount, const char ***listptr);

       Captured substrings can be  accessed  directly  by  using  the  offsets
       returned  by  pcre_exec()  in  ovector.  For convenience, the functions
       pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
       string_list()  are  provided for extracting captured substrings as new,
       separate, zero-terminated strings. These functions identify  substrings
       by  number.  The  next section describes functions for extracting named
       substrings.

       A substring that contains a binary zero is correctly extracted and  has
       a  further zero added on the end, but the result is not, of course, a C
       string.  However, you can process such a string  by  referring  to  the
       length  that  is  returned  by  pcre_copy_substring() and pcre_get_sub-
       string().  Unfortunately, the interface to pcre_get_substring_list() is
       not  adequate for handling strings containing binary zeros, because the
       end of the final string is not independently indicated.

       The first three arguments are the same for all  three  of  these  func-
       tions:  subject  is  the subject string that has just been successfully
       matched, ovector is a pointer to the vector of integer offsets that was
       passed to pcre_exec(), and stringcount is the number of substrings that
       were captured by the match, including the substring  that  matched  the
       entire regular expression. This is the value returned by pcre_exec() if
       it is greater than zero. If pcre_exec() returned zero, indicating  that
       it  ran out of space in ovector, the value passed as stringcount should
       be the number of elements in the vector divided by three.

       The functions pcre_copy_substring() and pcre_get_substring() extract  a
       single  substring,  whose  number  is given as stringnumber. A value of
       zero extracts the substring that matched the  entire  pattern,  whereas
       higher  values  extract  the  captured  substrings.  For pcre_copy_sub-
       string(), the string is placed in buffer,  whose  length  is  given  by
       buffersize,  while  for  pcre_get_substring()  a new block of memory is
       obtained via pcre_malloc, and its address is  returned  via  stringptr.
       The  yield  of  the function is the length of the string, not including
       the terminating zero, or one of these error codes:

         PCRE_ERROR_NOMEMORY       (-6)

       The buffer was too small for pcre_copy_substring(), or the  attempt  to
       get memory failed for pcre_get_substring().

         PCRE_ERROR_NOSUBSTRING    (-7)

       There is no substring whose number is stringnumber.

       The  pcre_get_substring_list()  function  extracts  all  available sub-
       strings and builds a list of pointers to them. All this is  done  in  a
       single block of memory that is obtained via pcre_malloc. The address of
       the memory block is returned via listptr, which is also  the  start  of
       the  list  of  string pointers. The end of the list is marked by a NULL
       pointer. The yield of the function is zero if all  went  well,  or  the
       error code

         PCRE_ERROR_NOMEMORY       (-6)

       if the attempt to get the memory block failed.

       When  any of these functions encounter a substring that is unset, which
       can happen when capturing subpattern number n+1 matches  some  part  of
       the  subject, but subpattern n has not been used at all, they return an
       empty string. This can be distinguished from a genuine zero-length sub-
       string  by inspecting the appropriate offset in ovector, which is nega-
       tive for unset substrings.

       The two convenience functions pcre_free_substring() and  pcre_free_sub-
       string_list()  can  be  used  to free the memory returned by a previous
       call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
       tively.  They  do  nothing  more  than  call the function pointed to by
       pcre_free, which of course could be called directly from a  C  program.
       However,  PCRE is used in some situations where it is linked via a spe-
       cial  interface  to  another  programming  language  that  cannot   use
       pcre_free  directly;  it is for these cases that the functions are pro-
       vided.


EXTRACTING CAPTURED SUBSTRINGS BY NAME

       int pcre_get_stringnumber(const pcre *code,
            const char *name);

       int pcre_copy_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            char *buffer, int buffersize);

       int pcre_get_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            const char **stringptr);

       To extract a substring by name, you first have to find associated  num-
       ber.  For example, for this pattern

         (a+)b(?<xxx>\d+)...

       the number of the subpattern called "xxx" is 2. If the name is known to
       be unique (PCRE_DUPNAMES was not set), you can find the number from the
       name by calling pcre_get_stringnumber(). The first argument is the com-
       piled pattern, and the second is the name. The yield of the function is
       the  subpattern  number,  or PCRE_ERROR_NOSUBSTRING (-7) if there is no
       subpattern of that name.

       Given the number, you can extract the substring directly, or use one of
       the functions described in the previous section. For convenience, there
       are also two functions that do the whole job.

       Most   of   the   arguments    of    pcre_copy_named_substring()    and
       pcre_get_named_substring()  are  the  same  as  those for the similarly
       named functions that extract by number. As these are described  in  the
       previous  section,  they  are not re-described here. There are just two
       differences:

       First, instead of a substring number, a substring name is  given.  Sec-
       ond, there is an extra argument, given at the start, which is a pointer
       to the compiled pattern. This is needed in order to gain access to  the
       name-to-number translation table.

       These  functions call pcre_get_stringnumber(), and if it succeeds, they
       then call pcre_copy_substring() or pcre_get_substring(),  as  appropri-
       ate.  NOTE:  If PCRE_DUPNAMES is set and there are duplicate names, the
       behaviour may not be what you want (see the next section).

       Warning: If the pattern uses the (?| feature to set up multiple subpat-
       terns  with  the  same number, as described in the section on duplicate
       subpattern numbers in the pcrepattern page, you  cannot  use  names  to
       distinguish  the  different subpatterns, because names are not included
       in the compiled code. The matching process uses only numbers. For  this
       reason,  the  use of different names for subpatterns of the same number
       causes an error at compile time.


DUPLICATE SUBPATTERN NAMES

       int pcre_get_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

       When a pattern is compiled with the  PCRE_DUPNAMES  option,  names  for
       subpatterns  are not required to be unique. (Duplicate names are always
       allowed for subpatterns with the same number, created by using the  (?|
       feature.  Indeed,  if  such subpatterns are named, they are required to
       use the same names.)

       Normally, patterns with duplicate names are such that in any one match,
       only  one of the named subpatterns participates. An example is shown in
       the pcrepattern documentation.

       When   duplicates   are   present,   pcre_copy_named_substring()    and
       pcre_get_named_substring()  return the first substring corresponding to
       the given name that is set. If  none  are  set,  PCRE_ERROR_NOSUBSTRING
       (-7)  is  returned;  no  data  is returned. The pcre_get_stringnumber()
       function returns one of the numbers that are associated with the  name,
       but it is not defined which it is.

       If  you want to get full details of all captured substrings for a given
       name, you must use  the  pcre_get_stringtable_entries()  function.  The
       first argument is the compiled pattern, and the second is the name. The
       third and fourth are pointers to variables which  are  updated  by  the
       function. After it has run, they point to the first and last entries in
       the name-to-number table  for  the  given  name.  The  function  itself
       returns  the  length  of  each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
       there are none. The format of the table is described above in the  sec-
       tion  entitled  Information about a pattern above.  Given all the rele-
       vant entries for the name, you can extract each of their  numbers,  and
       hence the captured data, if any.


FINDING ALL POSSIBLE MATCHES

       The  traditional  matching  function  uses a similar algorithm to Perl,
       which stops when it finds the first match, starting at a given point in
       the  subject.  If you want to find all possible matches, or the longest
       possible match, consider using the alternative matching  function  (see
       below)  instead.  If you cannot use the alternative function, but still
       need to find all possible matches, you can kludge it up by  making  use
       of the callout facility, which is described in the pcrecallout documen-
       tation.

       What you have to do is to insert a callout right at the end of the pat-
       tern.   When your callout function is called, extract and save the cur-
       rent matched substring. Then return  1,  which  forces  pcre_exec()  to
       backtrack  and  try other alternatives. Ultimately, when it runs out of
       matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.


OBTAINING AN ESTIMATE OF STACK USAGE

       Matching certain patterns using pcre_exec() can use a  lot  of  process
       stack,  which  in  certain  environments can be rather limited in size.
       Some users find it helpful to have an estimate of the amount  of  stack
       that  is  used  by  pcre_exec(),  to help them set recursion limits, as
       described in the pcrestack documentation. The estimate that  is  output
       by pcretest when called with the -m and -C options is obtained by call-
       ing pcre_exec with the values NULL, NULL, NULL, -999, and -999 for  its
       first five arguments.

       Normally,  if  its  first  argument  is  NULL,  pcre_exec() immediately
       returns the negative error code PCRE_ERROR_NULL, but with this  special
       combination  of  arguments,  it returns instead a negative number whose
       absolute value is the approximate stack frame size in bytes.  (A  nega-
       tive  number  is  used so that it is clear that no match has happened.)
       The value is approximate because in  some  cases,  recursive  calls  to
       pcre_exec() occur when there are one or two additional variables on the
       stack.

       If PCRE has been compiled to use the heap  instead  of  the  stack  for
       recursion,  the  value  returned  is  the  size  of  each block that is
       obtained from the heap.


MATCHING A PATTERN: THE ALTERNATIVE FUNCTION

       int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);

       The function pcre_dfa_exec()  is  called  to  match  a  subject  string
       against  a  compiled pattern, using a matching algorithm that scans the
       subject string just once, and does not backtrack.  This  has  different
       characteristics  to  the  normal  algorithm, and is not compatible with
       Perl. Some of the features of PCRE patterns are not  supported.  Never-
       theless,  there are times when this kind of matching can be useful. For
       a discussion of the two matching algorithms, and  a  list  of  features
       that  pcre_dfa_exec() does not support, see the pcrematching documenta-
       tion.

       The arguments for the pcre_dfa_exec() function  are  the  same  as  for
       pcre_exec(), plus two extras. The ovector argument is used in a differ-
       ent way, and this is described below. The other  common  arguments  are
       used  in  the  same way as for pcre_exec(), so their description is not
       repeated here.

       The two additional arguments provide workspace for  the  function.  The
       workspace  vector  should  contain at least 20 elements. It is used for
       keeping  track  of  multiple  paths  through  the  pattern  tree.  More
       workspace  will  be  needed for patterns and subjects where there are a
       lot of potential matches.

       Here is an example of a simple call to pcre_dfa_exec():

         int rc;
         int ovector[10];
         int wspace[20];
         rc = pcre_dfa_exec(
           re,             /* result of pcre_compile() */
           NULL,           /* we didn't study the pattern */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           ovector,        /* vector of integers for substring information */
           10,             /* number of elements (NOT size in bytes) */
           wspace,         /* working space vector */
           20);            /* number of elements (NOT size in bytes) */

   Option bits for pcre_dfa_exec()

       The unused bits of the options argument  for  pcre_dfa_exec()  must  be
       zero.  The  only  bits  that  may  be  set are PCRE_ANCHORED, PCRE_NEW-
       LINE_xxx,        PCRE_NOTBOL,        PCRE_NOTEOL,        PCRE_NOTEMPTY,
       PCRE_NOTEMPTY_ATSTART,       PCRE_NO_UTF8_CHECK,      PCRE_BSR_ANYCRLF,
       PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD,  PCRE_PAR-
       TIAL_SOFT,  PCRE_DFA_SHORTEST,  and PCRE_DFA_RESTART.  All but the last
       four of these are  exactly  the  same  as  for  pcre_exec(),  so  their
       description is not repeated here.

         PCRE_PARTIAL_HARD
         PCRE_PARTIAL_SOFT

       These  have the same general effect as they do for pcre_exec(), but the
       details are slightly  different.  When  PCRE_PARTIAL_HARD  is  set  for
       pcre_dfa_exec(),  it  returns PCRE_ERROR_PARTIAL if the end of the sub-
       ject is reached and there is still at least  one  matching  possibility
       that requires additional characters. This happens even if some complete
       matches have also been found. When PCRE_PARTIAL_SOFT is set, the return
       code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end
       of the subject is reached, there have been  no  complete  matches,  but
       there  is  still  at least one matching possibility. The portion of the
       string that was inspected when the longest partial match was  found  is
       set  as  the  first  matching  string  in  both cases.  There is a more
       detailed discussion of partial and multi-segment matching,  with  exam-
       ples, in the pcrepartial documentation.

         PCRE_DFA_SHORTEST

       Setting  the  PCRE_DFA_SHORTEST option causes the matching algorithm to
       stop as soon as it has found one match. Because of the way the alterna-
       tive  algorithm  works, this is necessarily the shortest possible match
       at the first possible matching point in the subject string.

         PCRE_DFA_RESTART

       When pcre_dfa_exec() returns a partial match, it is possible to call it
       again,  with  additional  subject characters, and have it continue with
       the same match. The PCRE_DFA_RESTART option requests this action;  when
       it  is  set,  the workspace and wscount options must reference the same
       vector as before because data about the match so far is  left  in  them
       after a partial match. There is more discussion of this facility in the
       pcrepartial documentation.

   Successful returns from pcre_dfa_exec()

       When pcre_dfa_exec() succeeds, it may have matched more than  one  sub-
       string in the subject. Note, however, that all the matches from one run
       of the function start at the same point in  the  subject.  The  shorter
       matches  are all initial substrings of the longer matches. For example,
       if the pattern

         <.*>

       is matched against the string

         This is <something> <something else> <something further> no more

       the three matched strings are

         <something>
         <something> <something else>
         <something> <something else> <something further>

       On success, the yield of the function is a number  greater  than  zero,
       which  is  the  number of matched substrings. The substrings themselves
       are returned in ovector. Each string uses two elements;  the  first  is
       the  offset  to  the start, and the second is the offset to the end. In
       fact, all the strings have the same start  offset.  (Space  could  have
       been  saved by giving this only once, but it was decided to retain some
       compatibility with the way pcre_exec() returns data,  even  though  the
       meaning of the strings is different.)

       The strings are returned in reverse order of length; that is, the long-
       est matching string is given first. If there were too many  matches  to
       fit  into ovector, the yield of the function is zero, and the vector is
       filled with the longest matches.  Unlike  pcre_exec(),  pcre_dfa_exec()
       can use the entire ovector for returning matched strings.

   Error returns from pcre_dfa_exec()

       The  pcre_dfa_exec()  function returns a negative number when it fails.
       Many of the errors are the same  as  for  pcre_exec(),  and  these  are
       described  above.   There are in addition the following errors that are
       specific to pcre_dfa_exec():

         PCRE_ERROR_DFA_UITEM      (-16)

       This return is given if pcre_dfa_exec() encounters an item in the  pat-
       tern  that  it  does not support, for instance, the use of \C or a back
       reference.

         PCRE_ERROR_DFA_UCOND      (-17)

       This return is given if pcre_dfa_exec()  encounters  a  condition  item
       that  uses  a back reference for the condition, or a test for recursion
       in a specific group. These are not supported.

         PCRE_ERROR_DFA_UMLIMIT    (-18)

       This return is given if pcre_dfa_exec() is called with an  extra  block
       that  contains  a  setting  of the match_limit or match_limit_recursion
       fields. This is not supported (these fields  are  meaningless  for  DFA
       matching).

         PCRE_ERROR_DFA_WSSIZE     (-19)

       This  return  is  given  if  pcre_dfa_exec()  runs  out of space in the
       workspace vector.

         PCRE_ERROR_DFA_RECURSE    (-20)

       When a recursive subpattern is processed, the matching  function  calls
       itself  recursively,  using  private vectors for ovector and workspace.
       This error is given if the output vector  is  not  large  enough.  This
       should be extremely rare, as a vector of size 1000 is used.

         PCRE_ERROR_DFA_BADRESTART (-30)

       When  pcre_dfa_exec()  is called with the PCRE_DFA_RESTART option, some
       plausibility checks are made on the contents of  the  workspace,  which
       should  contain  data about the previous partial match. If any of these
       checks fail, this error is given.


SEE ALSO

       pcre16(3),  pcrebuild(3),  pcrecallout(3),  pcrecpp(3)(3),   pcrematch-
       ing(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcresample(3),
       pcrestack(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 04 May 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRECALLOUT(3)                                                  PCRECALLOUT(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE CALLOUTS

       int (*pcre_callout)(pcre_callout_block *);

       int (*pcre16_callout)(pcre16_callout_block *);

       PCRE provides a feature called "callout", which is a means of temporar-
       ily passing control to the caller of PCRE  in  the  middle  of  pattern
       matching.  The  caller of PCRE provides an external function by putting
       its entry point in the global variable pcre_callout (pcre16_callout for
       the  16-bit  library).  By  default, this variable contains NULL, which
       disables all calling out.

       Within a regular expression, (?C) indicates the  points  at  which  the
       external  function  is  to  be  called. Different callout points can be
       identified by putting a number less than 256 after the  letter  C.  The
       default  value  is  zero.   For  example,  this pattern has two callout
       points:

         (?C1)abc(?C2)def

       If the PCRE_AUTO_CALLOUT option bit is set when a pattern is  compiled,
       PCRE  automatically  inserts callouts, all with number 255, before each
       item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the
       pattern

         A(\d{2}|--)

       it is processed as if it were

       (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

       Notice  that  there  is a callout before and after each parenthesis and
       alternation bar. Automatic  callouts  can  be  used  for  tracking  the
       progress  of  pattern matching. The pcretest command has an option that
       sets automatic callouts; when it is used, the output indicates how  the
       pattern  is  matched. This is useful information when you are trying to
       optimize the performance of a particular pattern.

       The use of callouts in a pattern makes it ineligible  for  optimization
       by  the  just-in-time  compiler.  Studying  such  a  pattern  with  the
       PCRE_STUDY_JIT_COMPILE option always fails.


MISSING CALLOUTS

       You should be aware that, because of  optimizations  in  the  way  PCRE
       matches  patterns  by  default,  callouts  sometimes do not happen. For
       example, if the pattern is

         ab(?C4)cd

       PCRE knows that any matching string must contain the letter "d". If the
       subject  string  is "abyz", the lack of "d" means that matching doesn't
       ever start, and the callout is never  reached.  However,  with  "abyd",
       though the result is still no match, the callout is obeyed.

       If  the pattern is studied, PCRE knows the minimum length of a matching
       string, and will immediately give a "no match" return without  actually
       running  a  match if the subject is not long enough, or, for unanchored
       patterns, if it has been scanned far enough.

       You can disable these optimizations by passing the  PCRE_NO_START_OPTI-
       MIZE  option  to the matching function, or by starting the pattern with
       (*NO_START_OPT). This slows down the matching process, but does  ensure
       that callouts such as the example above are obeyed.


THE CALLOUT INTERFACE

       During  matching, when PCRE reaches a callout point, the external func-
       tion defined by pcre_callout or pcre16_callout  is  called  (if  it  is
       set).   This applies to both normal and DFA matching. The only argument
       to the callout function is a pointer to a pcre_callout or  pcre16_call-
       out block.  These structures contains the following fields:

         int           version;
         int           callout_number;
         int          *offset_vector;
         const char   *subject;           (8-bit version)
         PCRE_SPTR16   subject;           (16-bit version)
         int           subject_length;
         int           start_match;
         int           current_position;
         int           capture_top;
         int           capture_last;
         void         *callout_data;
         int           pattern_position;
         int           next_item_length;
         const unsigned char *mark;       (8-bit version)
         const PCRE_UCHAR16  *mark;       (16-bit version)

       The  version  field  is an integer containing the version number of the
       block format. The initial version was 0; the current version is 2.  The
       version  number  will  change  again in future if additional fields are
       added, but the intention is never to remove any of the existing fields.

       The callout_number field contains the number of the  callout,  as  com-
       piled  into  the pattern (that is, the number after ?C for manual call-
       outs, and 255 for automatically generated callouts).

       The offset_vector field is a pointer to the vector of offsets that  was
       passed  by  the  caller  to  the matching function. When pcre_exec() or
       pcre16_exec() is used, the contents  can  be  inspected,  in  order  to
       extract  substrings  that  have been matched so far, in the same way as
       for extracting substrings after a match  has  completed.  For  the  DFA
       matching functions, this field is not useful.

       The subject and subject_length fields contain copies of the values that
       were passed to the matching function.

       The start_match field normally contains the offset within  the  subject
       at  which  the  current  match  attempt started. However, if the escape
       sequence \K has been encountered, this value is changed to reflect  the
       modified  starting  point.  If the pattern is not anchored, the callout
       function may be called several times from the same point in the pattern
       for different starting points in the subject.

       The  current_position  field  contains the offset within the subject of
       the current match pointer.

       When the pcre_exec() or pcre16_exec() is used,  the  capture_top  field
       contains one more than the number of the highest numbered captured sub-
       string so far. If no substrings have been captured, the value  of  cap-
       ture_top  is  one.  This  is always the case when the DFA functions are
       used, because they do not support captured substrings.

       The capture_last field contains the number of the  most  recently  cap-
       tured  substring. If no substrings have been captured, its value is -1.
       This is always the case for the DFA matching functions.

       The callout_data field contains a value that is passed  to  a  matching
       function  specifically so that it can be passed back in callouts. It is
       passed in the callout_data field of a pcre_extra or  pcre16_extra  data
       structure.  If  no such data was passed, the value of callout_data in a
       callout block is NULL. There is a description of the pcre_extra  struc-
       ture in the pcreapi documentation.

       The  pattern_position  field  is  present from version 1 of the callout
       structure. It contains the offset to the next item to be matched in the
       pattern string.

       The  next_item_length  field  is  present from version 1 of the callout
       structure. It contains the length of the next item to be matched in the
       pattern  string.  When  the callout immediately precedes an alternation
       bar, a closing parenthesis, or the end of the pattern,  the  length  is
       zero.  When  the callout precedes an opening parenthesis, the length is
       that of the entire subpattern.

       The pattern_position and next_item_length fields are intended  to  help
       in  distinguishing between different automatic callouts, which all have
       the same callout number. However, they are set for all callouts.

       The mark field is present from version 2 of the callout  structure.  In
       callouts from pcre_exec() or pcre16_exec() it contains a pointer to the
       zero-terminated name of the most recently passed (*MARK), (*PRUNE),  or
       (*THEN)  item  in the match, or NULL if no such items have been passed.
       Instances of (*PRUNE) or (*THEN) without a name  do  not  obliterate  a
       previous  (*MARK).  In  callouts  from  the DFA matching functions this
       field always contains NULL.


RETURN VALUES

       The external callout function returns an integer to PCRE. If the  value
       is  zero,  matching  proceeds  as  normal. If the value is greater than
       zero, matching fails at the current point, but  the  testing  of  other
       matching possibilities goes ahead, just as if a lookahead assertion had
       failed. If the value is less than zero, the  match  is  abandoned,  the
       matching function returns the negative value.

       Negative   values   should   normally   be   chosen  from  the  set  of
       PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
       dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
       reserved for use by callout functions; it will never be  used  by  PCRE
       itself.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 08 Janurary 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRECOMPAT(3)                                                    PCRECOMPAT(3)


NAME
       PCRE - Perl-compatible regular expressions


DIFFERENCES BETWEEN PCRE AND PERL

       This  document describes the differences in the ways that PCRE and Perl
       handle regular expressions. The differences  described  here  are  with
       respect to Perl versions 5.10 and above.

       1. PCRE has only a subset of Perl's Unicode support. Details of what it
       does have are given in the pcreunicode page.

       2. PCRE allows repeat quantifiers only on parenthesized assertions, but
       they  do  not mean what you might think. For example, (?!a){3} does not
       assert that the next three characters are not "a". It just asserts that
       the next character is not "a" three times (in principle: PCRE optimizes
       this to run the assertion just once). Perl allows repeat quantifiers on
       other assertions such as \b, but these do not seem to have any use.

       3.  Capturing  subpatterns  that occur inside negative lookahead asser-
       tions are counted, but their entries in the offsets  vector  are  never
       set.  Perl sets its numerical variables from any such patterns that are
       matched before the assertion fails to match something (thereby succeed-
       ing),  but  only  if the negative lookahead assertion contains just one
       branch.

       4. Though binary zero characters are supported in the  subject  string,
       they are not allowed in a pattern string because it is passed as a nor-
       mal C string, terminated by zero. The escape sequence \0 can be used in
       the pattern to represent a binary zero.

       5.  The  following Perl escape sequences are not supported: \l, \u, \L,
       \U, and \N when followed by a character name or Unicode value.  (\N  on
       its own, matching a non-newline character, is supported.) In fact these
       are implemented by Perl's general string-handling and are not  part  of
       its  pattern  matching engine. If any of these are encountered by PCRE,
       an error is generated by default. However, if the  PCRE_JAVASCRIPT_COM-
       PAT  option  is set, \U and \u are interpreted as JavaScript interprets
       them.

       6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE
       is  built  with Unicode character property support. The properties that
       can be tested with \p and \P are limited to the general category  prop-
       erties  such  as  Lu and Nd, script names such as Greek or Han, and the
       derived properties Any and L&. PCRE does  support  the  Cs  (surrogate)
       property,  which  Perl  does  not; the Perl documentation says "Because
       Perl hides the need for the user to understand the internal representa-
       tion  of Unicode characters, there is no need to implement the somewhat
       messy concept of surrogates."

       7. PCRE implements a simpler version of \X than Perl, which changed  to
       make  \X  match what Unicode calls an "extended grapheme cluster". This
       is more complicated than an extended Unicode sequence,  which  is  what
       PCRE matches.

       8. PCRE does support the \Q...\E escape for quoting substrings. Charac-
       ters in between are treated as literals.  This  is  slightly  different
       from  Perl  in  that  $  and  @ are also handled as literals inside the
       quotes. In Perl, they cause variable interpolation (but of course  PCRE
       does not have variables). Note the following examples:

           Pattern            PCRE matches      Perl matches

           \Qabc$xyz\E        abc$xyz           abc followed by the
                                                  contents of $xyz
           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz

       The  \Q...\E  sequence  is recognized both inside and outside character
       classes.

       9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
       constructions.  However,  there is support for recursive patterns. This
       is not available in Perl 5.8, but it is in Perl 5.10.  Also,  the  PCRE
       "callout"  feature allows an external function to be called during pat-
       tern matching. See the pcrecallout documentation for details.

       10. Subpatterns that are called as subroutines (whether or  not  recur-
       sively)  are  always  treated  as  atomic  groups in PCRE. This is like
       Python, but unlike Perl.  Captured values that are set outside  a  sub-
       routine  call  can  be  reference from inside in PCRE, but not in Perl.
       There is a discussion that explains these differences in more detail in
       the section on recursion differences from Perl in the pcrepattern page.

       11.  If  any of the backtracking control verbs are used in an assertion
       or in a subpattern that is called  as  a  subroutine  (whether  or  not
       recursively),  their effect is confined to that subpattern; it does not
       extend to the surrounding pattern. This is not always the case in Perl.
       In  particular,  if  (*THEN)  is present in a group that is called as a
       subroutine, its action is limited to that group, even if the group does
       not  contain any | characters. There is one exception to this: the name
       from a *(MARK), (*PRUNE), or (*THEN) that is encountered in a  success-
       ful  positive  assertion  is passed back when a match succeeds (compare
       capturing parentheses in assertions). Note that  such  subpatterns  are
       processed as anchored at the point where they are tested.

       12.  There are some differences that are concerned with the settings of
       captured strings when part of  a  pattern  is  repeated.  For  example,
       matching  "aba"  against  the  pattern  /^(a(b)?)+$/  in Perl leaves $2
       unset, but in PCRE it is set to "b".

       13. PCRE's handling of duplicate subpattern numbers and duplicate  sub-
       pattern names is not as general as Perl's. This is a consequence of the
       fact the PCRE works internally just with numbers, using an external ta-
       ble  to  translate  between numbers and names. In particular, a pattern
       such as (?|(?<a>A)|(?<b)B), where the two  capturing  parentheses  have
       the  same  number  but different names, is not supported, and causes an
       error at compile time. If it were allowed, it would not be possible  to
       distinguish  which  parentheses matched, because both names map to cap-
       turing subpattern number 1. To avoid this confusing situation, an error
       is given at compile time.

       14.  Perl  recognizes  comments  in some places that PCRE does not, for
       example, between the ( and ? at the start of a subpattern.  If  the  /x
       modifier is set, Perl allows white space between ( and ? but PCRE never
       does, even if the PCRE_EXTENDED option is set.

       15. PCRE provides some extensions to the Perl regular expression facil-
       ities.   Perl  5.10  includes new features that are not in earlier ver-
       sions of Perl, some of which (such as named parentheses) have  been  in
       PCRE for some time. This list is with respect to Perl 5.10:

       (a)  Although  lookbehind  assertions  in  PCRE must match fixed length
       strings, each alternative branch of a lookbehind assertion can match  a
       different  length  of  string.  Perl requires them all to have the same
       length.

       (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the  $
       meta-character matches only at the very end of the string.

       (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
       cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
       ignored.  (Perl can be made to issue a warning.)

       (d)  If  PCRE_UNGREEDY is set, the greediness of the repetition quanti-
       fiers is inverted, that is, by default they are not greedy, but if fol-
       lowed by a question mark they are.

       (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
       tried only at the first matching position in the subject string.

       (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
       and  PCRE_NO_AUTO_CAPTURE  options for pcre_exec() have no Perl equiva-
       lents.

       (g) The \R escape sequence can be restricted to match only CR,  LF,  or
       CRLF by the PCRE_BSR_ANYCRLF option.

       (h) The callout facility is PCRE-specific.

       (i) The partial matching facility is PCRE-specific.

       (j) Patterns compiled by PCRE can be saved and re-used at a later time,
       even on different hosts that have the other endianness.  However,  this
       does not apply to optimized data created by the just-in-time compiler.

       (k)   The   alternative   matching   functions   (pcre_dfa_exec()   and
       pcre16_dfa_exec()) match in a different way and are  not  Perl-compati-
       ble.

       (l)  PCRE  recognizes some special sequences such as (*CR) at the start
       of a pattern that set overall options that cannot be changed within the
       pattern.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 01 June 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREPATTERN(3)                                                  PCREPATTERN(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE REGULAR EXPRESSION DETAILS

       The  syntax and semantics of the regular expressions that are supported
       by PCRE are described in detail below. There is a quick-reference  syn-
       tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
       semantics as closely as it can. PCRE  also  supports  some  alternative
       regular  expression  syntax (which does not conflict with the Perl syn-
       tax) in order to provide some compatibility with regular expressions in
       Python, .NET, and Oniguruma.

       Perl's  regular expressions are described in its own documentation, and
       regular expressions in general are covered in a number of  books,  some
       of  which  have  copious  examples. Jeffrey Friedl's "Mastering Regular
       Expressions", published by  O'Reilly,  covers  regular  expressions  in
       great  detail.  This  description  of  PCRE's  regular  expressions  is
       intended as reference material.

       The original operation of PCRE was on strings of  one-byte  characters.
       However,  there  is  now also support for UTF-8 strings in the original
       library, and a second library that supports 16-bit and UTF-16 character
       strings. To use these features, PCRE must be built to include appropri-
       ate support. When using UTF strings you must either call the  compiling
       function  with  the PCRE_UTF8 or PCRE_UTF16 option, or the pattern must
       start with one of these special sequences:

         (*UTF8)
         (*UTF16)

       Starting a pattern with such a sequence is equivalent  to  setting  the
       relevant option. This feature is not Perl-compatible. How setting a UTF
       mode affects pattern matching is mentioned  in  several  places  below.
       There is also a summary of features in the pcreunicode page.

       Another  special  sequence that may appear at the start of a pattern or
       in combination with (*UTF8) or (*UTF16) is:

         (*UCP)

       This has the same effect as setting  the  PCRE_UCP  option:  it  causes
       sequences  such  as  \d  and  \w to use Unicode properties to determine
       character types, instead of recognizing only characters with codes less
       than 128 via a lookup table.

       If  a  pattern  starts  with (*NO_START_OPT), it has the same effect as
       setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
       time. There are also some more of these special sequences that are con-
       cerned with the handling of newlines; they are described below.

       The remainder of this document discusses the  patterns  that  are  sup-
       ported  by  PCRE  when  one  its  main  matching functions, pcre_exec()
       (8-bit) or pcre16_exec() (16-bit), is used. PCRE also  has  alternative
       matching  functions, pcre_dfa_exec() and pcre16_dfa_exec(), which match
       using a different algorithm that is not Perl-compatible.  Some  of  the
       features  discussed  below are not available when DFA matching is used.
       The advantages and disadvantages of the alternative functions, and  how
       they  differ from the normal functions, are discussed in the pcrematch-
       ing page.


NEWLINE CONVENTIONS

       PCRE supports five different conventions for indicating line breaks  in
       strings:  a  single  CR (carriage return) character, a single LF (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding,  or  any Unicode newline sequence. The pcreapi page has further
       discussion about newlines, and shows how to set the newline  convention
       in the options arguments for the compiling and matching functions.

       It  is also possible to specify a newline convention by starting a pat-
       tern string with one of the following five sequences:

         (*CR)        carriage return
         (*LF)        linefeed
         (*CRLF)      carriage return, followed by linefeed
         (*ANYCRLF)   any of the three above
         (*ANY)       all Unicode newline sequences

       These override the default and the options given to the compiling func-
       tion.  For  example,  on  a Unix system where LF is the default newline
       sequence, the pattern

         (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no  longer  a  newline. Note that these special settings, which are not
       Perl-compatible, are recognized only at the very start  of  a  pattern,
       and  that  they  must  be  in  upper  case. If more than one of them is
       present, the last one is used.

       The newline convention affects the interpretation of the dot  metachar-
       acter  when  PCRE_DOTALL is not set, and also the behaviour of \N. How-
       ever, it does not affect  what  the  \R  escape  sequence  matches.  By
       default,  this is any Unicode newline sequence, for Perl compatibility.
       However, this can be changed; see the description of \R in the  section
       entitled  "Newline sequences" below. A change of \R setting can be com-
       bined with a change of newline convention.


CHARACTERS AND METACHARACTERS

       A regular expression is a pattern that is  matched  against  a  subject
       string  from  left  to right. Most characters stand for themselves in a
       pattern, and match the corresponding characters in the  subject.  As  a
       trivial example, the pattern

         The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless matching is specified (the PCRE_CASELESS option), letters  are
       matched  independently  of case. In a UTF mode, PCRE always understands
       the concept of case for characters whose values are less than  128,  so
       caseless  matching  is always possible. For characters with higher val-
       ues, the concept of case is supported if PCRE is compiled with  Unicode
       property  support,  but  not  otherwise.   If  you want to use caseless
       matching for characters 128 and above, you must  ensure  that  PCRE  is
       compiled with Unicode property support as well as with UTF support.

       The  power  of  regular  expressions  comes from the ability to include
       alternatives and repetitions in the pattern. These are encoded  in  the
       pattern by the use of metacharacters, which do not stand for themselves
       but instead are interpreted in some special way.

       There are two different sets of metacharacters: those that  are  recog-
       nized  anywhere in the pattern except within square brackets, and those
       that are recognized within square brackets.  Outside  square  brackets,
       the metacharacters are as follows:

         \      general escape character with several uses
         ^      assert start of string (or line, in multiline mode)
         $      assert end of string (or line, in multiline mode)
         .      match any character except newline (by default)
         [      start character class definition
         |      start of alternative branch
         (      start subpattern
         )      end subpattern
         ?      extends the meaning of (
                also 0 or 1 quantifier
                also quantifier minimizer
         *      0 or more quantifier
         +      1 or more quantifier
                also "possessive quantifier"
         {      start min/max quantifier

       Part  of  a  pattern  that is in square brackets is called a "character
       class". In a character class the only metacharacters are:

         \      general escape character
         ^      negate the class, but only if the first character
         -      indicates character range
         [      POSIX character class (only if followed by POSIX
                  syntax)
         ]      terminates the character class

       The following sections describe the use of each of the metacharacters.


BACKSLASH

       The backslash character has several uses. Firstly, if it is followed by
       a character that is not a number or a letter, it takes away any special
       meaning that character may have. This use of  backslash  as  an  escape
       character applies both inside and outside character classes.

       For  example,  if  you want to match a * character, you write \* in the
       pattern.  This escaping action applies whether  or  not  the  following
       character  would  otherwise be interpreted as a metacharacter, so it is
       always safe to precede a non-alphanumeric  with  backslash  to  specify
       that  it stands for itself. In particular, if you want to match a back-
       slash, you write \\.

       In a UTF mode, only ASCII numbers and letters have any special  meaning
       after  a  backslash.  All  other characters (in particular, those whose
       codepoints are greater than 127) are treated as literals.

       If a pattern is compiled with the PCRE_EXTENDED option, white space  in
       the  pattern (other than in a character class) and characters between a
       # outside a character class and the next newline are ignored. An escap-
       ing  backslash  can  be used to include a white space or # character as
       part of the pattern.

       If you want to remove the special meaning from a  sequence  of  charac-
       ters,  you can do so by putting them between \Q and \E. This is differ-
       ent from Perl in that $ and  @  are  handled  as  literals  in  \Q...\E
       sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola-
       tion. Note the following examples:

         Pattern            PCRE matches   Perl matches

         \Qabc$xyz\E        abc$xyz        abc followed by the
                                             contents of $xyz
         \Qabc\$xyz\E       abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes.   An  isolated \E that is not preceded by \Q is ignored. If \Q
       is not followed by \E later in the pattern, the literal  interpretation
       continues  to  the  end  of  the pattern (that is, \E is assumed at the
       end). If the isolated \Q is inside a character class,  this  causes  an
       error, because the character class is not terminated.

   Non-printing characters

       A second use of backslash provides a way of encoding non-printing char-
       acters in patterns in a visible manner. There is no restriction on  the
       appearance  of non-printing characters, apart from the binary zero that
       terminates a pattern, but when a pattern  is  being  prepared  by  text
       editing,  it  is  often  easier  to  use  one  of  the following escape
       sequences than the binary character it represents:

         \a        alarm, that is, the BEL character (hex 07)
         \cx       "control-x", where x is any ASCII character
         \e        escape (hex 1B)
         \f        form feed (hex 0C)
         \n        linefeed (hex 0A)
         \r        carriage return (hex 0D)
         \t        tab (hex 09)
         \ddd      character with octal code ddd, or back reference
         \xhh      character with hex code hh
         \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
         \uhhhh    character with hex code hhhh (JavaScript mode only)

       The precise effect of \cx is as follows: if x is a lower  case  letter,
       it  is converted to upper case. Then bit 6 of the character (hex 40) is
       inverted.  Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
       is  7B),  while  \c; becomes hex 7B (; is 3B). If the byte following \c
       has a value greater than 127, a compile-time error occurs.  This  locks
       out non-ASCII characters in all modes. (When PCRE is compiled in EBCDIC
       mode, all byte values are valid. A lower case letter  is  converted  to
       upper case, and then the 0xc0 bits are flipped.)

       By  default,  after  \x,  from  zero to two hexadecimal digits are read
       (letters can be in upper or lower case). Any number of hexadecimal dig-
       its may appear between \x{ and }, but the character code is constrained
       as follows:

         8-bit non-UTF mode    less than 0x100
         8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
         16-bit non-UTF mode   less than 0x10000
         16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint

       Invalid Unicode codepoints are the range  0xd800  to  0xdfff  (the  so-
       called "surrogate" codepoints).

       If  characters  other than hexadecimal digits appear between \x{ and },
       or if there is no terminating }, this form of escape is not recognized.
       Instead,  the  initial  \x  will  be interpreted as a basic hexadecimal
       escape, with no following digits, giving a  character  whose  value  is
       zero.

       If  the  PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
       is as just described only when it is followed by two  hexadecimal  dig-
       its.   Otherwise,  it  matches  a  literal "x" character. In JavaScript
       mode, support for code points greater than 256 is provided by \u, which
       must  be  followed  by  four hexadecimal digits; otherwise it matches a
       literal "u" character.

       Characters whose value is less than 256 can be defined by either of the
       two  syntaxes for \x (or by \u in JavaScript mode). There is no differ-
       ence in the way they are handled. For example, \xdc is exactly the same
       as \x{dc} (or \u00dc in JavaScript mode).

       After  \0  up  to two further octal digits are read. If there are fewer
       than two digits, just  those  that  are  present  are  used.  Thus  the
       sequence \0\x\07 specifies two binary zeros followed by a BEL character
       (code value 7). Make sure you supply two digits after the initial  zero
       if the pattern character that follows is itself an octal digit.

       The handling of a backslash followed by a digit other than 0 is compli-
       cated.  Outside a character class, PCRE reads it and any following dig-
       its  as  a  decimal  number. If the number is less than 10, or if there
       have been at least that many previous capturing left parentheses in the
       expression,  the  entire  sequence  is  taken  as  a  back reference. A
       description of how this works is given later, following the  discussion
       of parenthesized subpatterns.

       Inside  a  character  class, or if the decimal number is greater than 9
       and there have not been that many capturing subpatterns, PCRE  re-reads
       up to three octal digits following the backslash, and uses them to gen-
       erate a data character. Any subsequent digits stand for themselves. The
       value  of  the  character  is constrained in the same way as characters
       specified in hexadecimal.  For example:

         \040   is another way of writing a space
         \40    is the same, provided there are fewer than 40
                   previous capturing subpatterns
         \7     is always a back reference
         \11    might be a back reference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a back reference, otherwise the
                   character with octal code 113
         \377   might be a back reference, otherwise
                   the value 255 (decimal)
         \81    is either a back reference, or a binary zero
                   followed by the two characters "8" and "1"

       Note that octal values of 100 or greater must not be  introduced  by  a
       leading zero, because no more than three octal digits are ever read.

       All the sequences that define a single character value can be used both
       inside and outside character classes. In addition, inside  a  character
       class, \b is interpreted as the backspace character (hex 08).

       \N  is not allowed in a character class. \B, \R, and \X are not special
       inside a character class. Like  other  unrecognized  escape  sequences,
       they  are  treated  as  the  literal  characters  "B",  "R", and "X" by
       default, but cause an error if the PCRE_EXTRA option is set. Outside  a
       character class, these sequences have different meanings.

   Unsupported escape sequences

       In  Perl, the sequences \l, \L, \u, and \U are recognized by its string
       handler and used  to  modify  the  case  of  following  characters.  By
       default,  PCRE does not support these escape sequences. However, if the
       PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U"  character,  and
       \u can be used to define a character by code point, as described in the
       previous section.

   Absolute and relative back references

       The sequence \g followed by an unsigned or a negative  number,  option-
       ally  enclosed  in braces, is an absolute or relative back reference. A
       named back reference can be coded as \g{name}. Back references are dis-
       cussed later, following the discussion of parenthesized subpatterns.

   Absolute and relative subroutine calls

       For  compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an  alternative  syntax for referencing a subpattern as a "subroutine".
       Details are discussed later.   Note  that  \g{...}  (Perl  syntax)  and
       \g<...>  (Oniguruma  syntax)  are  not synonymous. The former is a back
       reference; the latter is a subroutine call.

   Generic character types

       Another use of backslash is for specifying generic character types:

         \d     any decimal digit
         \D     any character that is not a decimal digit
         \h     any horizontal white space character
         \H     any character that is not a horizontal white space character
         \s     any white space character
         \S     any character that is not a white space character
         \v     any vertical white space character
         \V     any character that is not a vertical white space character
         \w     any "word" character
         \W     any "non-word" character

       There is also the single sequence \N, which matches a non-newline char-
       acter.   This  is the same as the "." metacharacter when PCRE_DOTALL is
       not set. Perl also uses \N to match characters by name; PCRE  does  not
       support this.

       Each  pair of lower and upper case escape sequences partitions the com-
       plete set of characters into two disjoint  sets.  Any  given  character
       matches  one, and only one, of each pair. The sequences can appear both
       inside and outside character classes. They each match one character  of
       the  appropriate  type.  If the current matching point is at the end of
       the subject string, all of them fail, because there is no character  to
       match.

       For  compatibility  with Perl, \s does not match the VT character (code
       11).  This makes it different from the the POSIX "space" class. The  \s
       characters  are  HT  (9), LF (10), FF (12), CR (13), and space (32). If
       "use locale;" is included in a Perl script, \s may match the VT charac-
       ter. In PCRE, it never does.

       A  "word"  character is an underscore or any character that is a letter
       or digit.  By default, the definition of letters  and  digits  is  con-
       trolled  by PCRE's low-valued character tables, and may vary if locale-
       specific matching is taking place (see "Locale support" in the  pcreapi
       page).  For  example,  in  a French locale such as "fr_FR" in Unix-like
       systems, or "french" in Windows, some character codes greater than  128
       are  used  for  accented letters, and these are then matched by \w. The
       use of locales with Unicode is discouraged.

       By default, in a UTF mode, characters  with  values  greater  than  128
       never  match  \d,  \s,  or  \w,  and always match \D, \S, and \W. These
       sequences retain their original meanings from before  UTF  support  was
       available,  mainly for efficiency reasons. However, if PCRE is compiled
       with Unicode property support, and the PCRE_UCP option is set, the  be-
       haviour  is  changed  so  that Unicode properties are used to determine
       character types, as follows:

         \d  any character that \p{Nd} matches (decimal digit)
         \s  any character that \p{Z} matches, plus HT, LF, FF, CR
         \w  any character that \p{L} or \p{N} matches, plus underscore

       The upper case escapes match the inverse sets of characters. Note  that
       \d  matches  only decimal digits, whereas \w matches any Unicode digit,
       as well as any Unicode letter, and underscore. Note also that  PCRE_UCP
       affects  \b,  and  \B  because  they are defined in terms of \w and \W.
       Matching these sequences is noticeably slower when PCRE_UCP is set.

       The sequences \h, \H, \v, and \V are features that were added  to  Perl
       at  release  5.10. In contrast to the other sequences, which match only
       ASCII characters by default, these  always  match  certain  high-valued
       codepoints,  whether or not PCRE_UCP is set. The horizontal space char-
       acters are:

         U+0009     Horizontal tab
         U+0020     Space
         U+00A0     Non-break space
         U+1680     Ogham space mark
         U+180E     Mongolian vowel separator
         U+2000     En quad
         U+2001     Em quad
         U+2002     En space
         U+2003     Em space
         U+2004     Three-per-em space
         U+2005     Four-per-em space
         U+2006     Six-per-em space
         U+2007     Figure space
         U+2008     Punctuation space
         U+2009     Thin space
         U+200A     Hair space
         U+202F     Narrow no-break space
         U+205F     Medium mathematical space
         U+3000     Ideographic space

       The vertical space characters are:

         U+000A     Linefeed
         U+000B     Vertical tab
         U+000C     Form feed
         U+000D     Carriage return
         U+0085     Next line
         U+2028     Line separator
         U+2029     Paragraph separator

       In 8-bit, non-UTF-8 mode, only the characters with codepoints less than
       256 are relevant.

   Newline sequences

       Outside  a  character class, by default, the escape sequence \R matches
       any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is  equivalent
       to the following:

         (?>\r\n|\n|\x0b|\f|\r|\x85)

       This  is  an  example  of an "atomic group", details of which are given
       below.  This particular group matches either the two-character sequence
       CR  followed  by  LF,  or  one  of  the single characters LF (linefeed,
       U+000A), VT (vertical tab, U+000B), FF (form feed,  U+000C),  CR  (car-
       riage  return,  U+000D),  or NEL (next line, U+0085). The two-character
       sequence is treated as a single unit that cannot be split.

       In other modes, two additional characters whose codepoints are  greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
       rator, U+2029).  Unicode character property support is not  needed  for
       these characters to be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set  of  Unicode  line  endings)  by  setting  the  option
       PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
       (BSR is an abbrevation for "backslash R".) This can be made the default
       when  PCRE  is  built;  if this is the case, the other behaviour can be
       requested via the PCRE_BSR_UNICODE option.   It  is  also  possible  to
       specify  these  settings  by  starting a pattern string with one of the
       following sequences:

         (*BSR_ANYCRLF)   CR, LF, or CRLF only
         (*BSR_UNICODE)   any Unicode newline sequence

       These override the default and the options given to the compiling func-
       tion,  but  they  can  themselves  be  overridden by options given to a
       matching function. Note that these  special  settings,  which  are  not
       Perl-compatible,  are  recognized  only at the very start of a pattern,
       and that they must be in upper case.  If  more  than  one  of  them  is
       present,  the  last  one is used. They can be combined with a change of
       newline convention; for example, a pattern can start with:

         (*ANY)(*BSR_ANYCRLF)

       They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
       sequences.  Inside  a character class, \R is treated as an unrecognized
       escape sequence, and so matches the letter "R" by default,  but  causes
       an error if PCRE_EXTRA is set.

   Unicode character properties

       When PCRE is built with Unicode character property support, three addi-
       tional escape sequences that match characters with specific  properties
       are  available.   When  in 8-bit non-UTF-8 mode, these sequences are of
       course limited to testing characters whose  codepoints  are  less  than
       256, but they do work in this mode.  The extra escape sequences are:

         \p{xx}   a character with the xx property
         \P{xx}   a character without the xx property
         \X       an extended Unicode sequence

       The  property  names represented by xx above are limited to the Unicode
       script names, the general category properties, "Any", which matches any
       character   (including  newline),  and  some  special  PCRE  properties
       (described in the next section).  Other Perl properties such as  "InMu-
       sicalSymbols"  are  not  currently supported by PCRE. Note that \P{Any}
       does not match any characters, so always causes a match failure.

       Sets of Unicode characters are defined as belonging to certain scripts.
       A  character from one of these sets can be matched using a script name.
       For example:

         \p{Greek}
         \P{Han}

       Those that are not part of an identified script are lumped together  as
       "Common". The current list of scripts is:

       Arabic,  Armenian,  Avestan, Balinese, Bamum, Batak, Bengali, Bopomofo,
       Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian,  Chakma,
       Cham,  Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret,
       Devanagari,  Egyptian_Hieroglyphs,  Ethiopic,   Georgian,   Glagolitic,
       Gothic,  Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
       gana,  Imperial_Aramaic,  Inherited,  Inscriptional_Pahlavi,   Inscrip-
       tional_Parthian,   Javanese,   Kaithi,   Kannada,  Katakana,  Kayah_Li,
       Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B,  Lisu,  Lycian,
       Lydian,    Malayalam,    Mandaic,    Meetei_Mayek,    Meroitic_Cursive,
       Meroitic_Hieroglyphs,  Miao,  Mongolian,  Myanmar,  New_Tai_Lue,   Nko,
       Ogham,    Old_Italic,   Old_Persian,   Old_South_Arabian,   Old_Turkic,
       Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic,  Samari-
       tan,  Saurashtra,  Sharada,  Shavian, Sinhala, Sora_Sompeng, Sundanese,
       Syloti_Nagri, Syriac, Tagalog, Tagbanwa,  Tai_Le,  Tai_Tham,  Tai_Viet,
       Takri,  Tamil,  Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai,
       Yi.

       Each character has exactly one Unicode general category property, spec-
       ified  by a two-letter abbreviation. For compatibility with Perl, nega-
       tion can be specified by including a  circumflex  between  the  opening
       brace  and  the  property  name.  For  example,  \p{^Lu} is the same as
       \P{Lu}.

       If only one letter is specified with \p or \P, it includes all the gen-
       eral  category properties that start with that letter. In this case, in
       the absence of negation, the curly brackets in the escape sequence  are
       optional; these two examples have the same effect:

         \p{L}
         \pL

       The following general category property codes are supported:

         C     Other
         Cc    Control
         Cf    Format
         Cn    Unassigned
         Co    Private use
         Cs    Surrogate

         L     Letter
         Ll    Lower case letter
         Lm    Modifier letter
         Lo    Other letter
         Lt    Title case letter
         Lu    Upper case letter

         M     Mark
         Mc    Spacing mark
         Me    Enclosing mark
         Mn    Non-spacing mark

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

         P     Punctuation
         Pc    Connector punctuation
         Pd    Dash punctuation
         Pe    Close punctuation
         Pf    Final punctuation
         Pi    Initial punctuation
         Po    Other punctuation
         Ps    Open punctuation

         S     Symbol
         Sc    Currency symbol
         Sk    Modifier symbol
         Sm    Mathematical symbol
         So    Other symbol

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

       The  special property L& is also supported: it matches a character that
       has the Lu, Ll, or Lt property, in other words, a letter  that  is  not
       classified as a modifier or "other".

       The  Cs  (Surrogate)  property  applies only to characters in the range
       U+D800 to U+DFFF. Such characters are not valid in Unicode strings  and
       so  cannot  be  tested  by  PCRE, unless UTF validity checking has been
       turned   off   (see   the   discussion   of   PCRE_NO_UTF8_CHECK    and
       PCRE_NO_UTF16_CHECK  in the pcreapi page). Perl does not support the Cs
       property.

       The long synonyms for  property  names  that  Perl  supports  (such  as
       \p{Letter})  are  not  supported by PCRE, nor is it permitted to prefix
       any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned) prop-
       erty.  Instead, this property is assumed for any code point that is not
       in the Unicode table.

       Specifying caseless matching does not affect  these  escape  sequences.
       For example, \p{Lu} always matches only upper case letters.

       The  \X  escape  matches  any number of Unicode characters that form an
       extended Unicode sequence. \X is equivalent to

         (?>\PM\pM*)

       That is, it matches a character without the "mark"  property,  followed
       by  zero  or  more  characters with the "mark" property, and treats the
       sequence as an atomic group (see below).  Characters  with  the  "mark"
       property  are  typically  accents  that affect the preceding character.
       None of them have codepoints less than 256, so in 8-bit non-UTF-8  mode
       \X matches any one character.

       Note that recent versions of Perl have changed \X to match what Unicode
       calls an "extended grapheme cluster", which has a more complicated def-
       inition.

       Matching  characters  by Unicode property is not fast, because PCRE has
       to search a structure that contains  data  for  over  fifteen  thousand
       characters. That is why the traditional escape sequences such as \d and
       \w do not use Unicode properties in PCRE by  default,  though  you  can
       make  them do so by setting the PCRE_UCP option or by starting the pat-
       tern with (*UCP).

   PCRE's additional properties

       As well as the standard Unicode properties described  in  the  previous
       section,  PCRE supports four more that make it possible to convert tra-
       ditional escape sequences such as \w and \s and POSIX character classes
       to use Unicode properties. PCRE uses these non-standard, non-Perl prop-
       erties internally when PCRE_UCP is set. They are:

         Xan   Any alphanumeric character
         Xps   Any POSIX space character
         Xsp   Any Perl space character
         Xwd   Any Perl "word" character

       Xan matches characters that have either the L (letter) or the  N  (num-
       ber)  property. Xps matches the characters tab, linefeed, vertical tab,
       form feed, or carriage return, and any other character that has  the  Z
       (separator) property.  Xsp is the same as Xps, except that vertical tab
       is excluded. Xwd matches the same characters as Xan, plus underscore.

   Resetting the match start

       The escape sequence \K causes any previously matched characters not  to
       be included in the final matched sequence. For example, the pattern:

         foo\Kbar

       matches  "foobar",  but reports that it has matched "bar". This feature
       is similar to a lookbehind assertion (described  below).   However,  in
       this  case, the part of the subject before the real match does not have
       to be of fixed length, as lookbehind assertions do. The use of \K  does
       not  interfere  with  the setting of captured substrings.  For example,
       when the pattern

         (foo)\Kbar

       matches "foobar", the first substring is still set to "foo".

       Perl documents that the use  of  \K  within  assertions  is  "not  well
       defined".  In  PCRE,  \K  is  acted upon when it occurs inside positive
       assertions, but is ignored in negative assertions.

   Simple assertions

       The final use of backslash is for certain simple assertions. An  asser-
       tion  specifies a condition that has to be met at a particular point in
       a match, without consuming any characters from the subject string.  The
       use  of subpatterns for more complicated assertions is described below.
       The backslashed assertions are:

         \b     matches at a word boundary
         \B     matches when not at a word boundary
         \A     matches at the start of the subject
         \Z     matches at the end of the subject
                 also matches before a newline at the end of the subject
         \z     matches only at the end of the subject
         \G     matches at the first matching position in the subject

       Inside a character class, \b has a different meaning;  it  matches  the
       backspace  character.  If  any  other  of these assertions appears in a
       character class, by default it matches the corresponding literal  char-
       acter  (for  example,  \B  matches  the  letter  B).  However,  if  the
       PCRE_EXTRA option is set, an "invalid escape sequence" error is  gener-
       ated instead.

       A  word  boundary is a position in the subject string where the current
       character and the previous character do not both match \w or  \W  (i.e.
       one  matches  \w  and the other matches \W), or the start or end of the
       string if the first or last character matches \w,  respectively.  In  a
       UTF  mode,  the  meanings  of  \w  and \W can be changed by setting the
       PCRE_UCP option. When this is done, it also affects \b and \B.  Neither
       PCRE  nor  Perl has a separate "start of word" or "end of word" metase-
       quence. However, whatever follows \b normally determines which  it  is.
       For example, the fragment \ba matches "a" at the start of a word.

       The  \A,  \Z,  and \z assertions differ from the traditional circumflex
       and dollar (described in the next section) in that they only ever match
       at  the  very start and end of the subject string, whatever options are
       set. Thus, they are independent of multiline mode. These  three  asser-
       tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
       affect only the behaviour of the circumflex and dollar  metacharacters.
       However,  if the startoffset argument of pcre_exec() is non-zero, indi-
       cating that matching is to start at a point other than the beginning of
       the  subject,  \A  can never match. The difference between \Z and \z is
       that \Z matches before a newline at the end of the string as well as at
       the very end, whereas \z matches only at the end.

       The  \G assertion is true only when the current matching position is at
       the start point of the match, as specified by the startoffset  argument
       of  pcre_exec().  It  differs  from \A when the value of startoffset is
       non-zero. By calling pcre_exec() multiple times with appropriate  argu-
       ments, you can mimic Perl's /g option, and it is in this kind of imple-
       mentation where \G can be useful.

       Note, however, that PCRE's interpretation of \G, as the  start  of  the
       current match, is subtly different from Perl's, which defines it as the
       end of the previous match. In Perl, these can  be  different  when  the
       previously  matched  string was empty. Because PCRE does just one match
       at a time, it cannot reproduce this behaviour.

       If all the alternatives of a pattern begin with \G, the  expression  is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.


CIRCUMFLEX AND DOLLAR

       Outside a character class, in the default matching mode, the circumflex
       character  is  an  assertion  that is true only if the current matching
       point is at the start of the subject string. If the  startoffset  argu-
       ment  of  pcre_exec()  is  non-zero,  circumflex can never match if the
       PCRE_MULTILINE option is unset. Inside a  character  class,  circumflex
       has an entirely different meaning (see below).

       Circumflex  need  not be the first character of the pattern if a number
       of alternatives are involved, but it should be the first thing in  each
       alternative  in  which  it appears if the pattern is ever to match that
       branch. If all possible alternatives start with a circumflex, that  is,
       if  the  pattern  is constrained to match only at the start of the sub-
       ject, it is said to be an "anchored" pattern.  (There  are  also  other
       constructs that can cause a pattern to be anchored.)

       A  dollar  character  is  an assertion that is true only if the current
       matching point is at the end of  the  subject  string,  or  immediately
       before a newline at the end of the string (by default). Dollar need not
       be the last character of the pattern if a number  of  alternatives  are
       involved,  but  it  should  be  the last item in any branch in which it
       appears. Dollar has no special meaning in a character class.

       The meaning of dollar can be changed so that it  matches  only  at  the
       very  end  of  the string, by setting the PCRE_DOLLAR_ENDONLY option at
       compile time. This does not affect the \Z assertion.

       The meanings of the circumflex and dollar characters are changed if the
       PCRE_MULTILINE  option  is  set.  When  this  is the case, a circumflex
       matches immediately after internal newlines as well as at the start  of
       the  subject  string.  It  does not match after a newline that ends the
       string. A dollar matches before any newlines in the string, as well  as
       at  the very end, when PCRE_MULTILINE is set. When newline is specified
       as the two-character sequence CRLF, isolated CR and  LF  characters  do
       not indicate newlines.

       For  example, the pattern /^abc$/ matches the subject string "def\nabc"
       (where \n represents a newline) in multiline mode, but  not  otherwise.
       Consequently,  patterns  that  are anchored in single line mode because
       all branches start with ^ are not anchored in  multiline  mode,  and  a
       match  for  circumflex  is  possible  when  the startoffset argument of
       pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is  ignored  if
       PCRE_MULTILINE is set.

       Note  that  the sequences \A, \Z, and \z can be used to match the start
       and end of the subject in both modes, and if all branches of a  pattern
       start  with  \A it is always anchored, whether or not PCRE_MULTILINE is
       set.


FULL STOP (PERIOD, DOT) AND \N

       Outside a character class, a dot in the pattern matches any one charac-
       ter  in  the subject string except (by default) a character that signi-
       fies the end of a line.

       When a line ending is defined as a single character, dot never  matches
       that  character; when the two-character sequence CRLF is used, dot does
       not match CR if it is immediately followed  by  LF,  but  otherwise  it
       matches  all characters (including isolated CRs and LFs). When any Uni-
       code line endings are being recognized, dot does not match CR or LF  or
       any of the other line ending characters.

       The  behaviour  of  dot  with regard to newlines can be changed. If the
       PCRE_DOTALL option is set, a dot matches  any  one  character,  without
       exception. If the two-character sequence CRLF is present in the subject
       string, it takes two dots to match it.

       The handling of dot is entirely independent of the handling of  circum-
       flex  and  dollar,  the  only relationship being that they both involve
       newlines. Dot has no special meaning in a character class.

       The escape sequence \N behaves like  a  dot,  except  that  it  is  not
       affected  by  the  PCRE_DOTALL  option.  In other words, it matches any
       character except one that signifies the end of a line. Perl  also  uses
       \N to match characters by name; PCRE does not support this.


MATCHING A SINGLE DATA UNIT

       Outside  a character class, the escape sequence \C matches any one data
       unit, whether or not a UTF mode is set. In the 8-bit library, one  data
       unit  is  one byte; in the 16-bit library it is a 16-bit unit. Unlike a
       dot, \C always matches line-ending characters. The feature is  provided
       in  Perl  in  order  to match individual bytes in UTF-8 mode, but it is
       unclear how it can usefully be used. Because \C  breaks  up  characters
       into  individual  data  units,  matching one unit with \C in a UTF mode
       means that the rest of the string may start with a malformed UTF  char-
       acter.  This  has  undefined  results,  because PCRE assumes that it is
       dealing with valid UTF strings (and by default it checks  this  at  the
       start     of    processing    unless    the    PCRE_NO_UTF8_CHECK    or
       PCRE_NO_UTF16_CHECK option is used).

       PCRE does not allow \C to appear in  lookbehind  assertions  (described
       below)  in  a UTF mode, because this would make it impossible to calcu-
       late the length of the lookbehind.

       In general, the \C escape sequence is best avoided. However, one way of
       using  it that avoids the problem of malformed UTF characters is to use
       a lookahead to check the length of the next character, as in this  pat-
       tern,  which  could be used with a UTF-8 string (ignore white space and
       line breaks):

         (?| (?=[\x00-\x7f])(\C) |
             (?=[\x80-\x{7ff}])(\C)(\C) |
             (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
             (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

       A group that starts with (?| resets the capturing  parentheses  numbers
       in  each  alternative  (see  "Duplicate Subpattern Numbers" below). The
       assertions at the start of each branch check the next  UTF-8  character
       for  values  whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
       character's individual bytes are then captured by the appropriate  num-
       ber of groups.


SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial by default.  However, if the PCRE_JAVASCRIPT_COMPAT option is set,
       a lone closing square bracket causes a compile-time error. If a closing
       square  bracket  is required as a member of the class, it should be the
       first data character in the class  (after  an  initial  circumflex,  if
       present) or escaped with a backslash.

       A  character  class matches a single character in the subject. In a UTF
       mode, the character may be more than one  data  unit  long.  A  matched
       character must be in the set of characters defined by the class, unless
       the first character in the class definition is a circumflex,  in  which
       case the subject character must not be in the set defined by the class.
       If a circumflex is actually required as a member of the  class,  ensure
       it is not the first character, or escape it with a backslash.

       For  example, the character class [aeiou] matches any lower case vowel,
       while [^aeiou] matches any character that is not a  lower  case  vowel.
       Note that a circumflex is just a convenient notation for specifying the
       characters that are in the class by enumerating those that are  not.  A
       class  that starts with a circumflex is not an assertion; it still con-
       sumes a character from the subject string, and therefore  it  fails  if
       the current pointer is at the end of the string.

       In  UTF-8  (UTF-16)  mode,  characters  with  values  greater  than 255
       (0xffff) can be included in a class as a literal string of data  units,
       or by using the \x{ escaping mechanism.

       When  caseless  matching  is set, any letters in a class represent both
       their upper case and lower case versions, so for  example,  a  caseless
       [aeiou]  matches  "A"  as well as "a", and a caseless [^aeiou] does not
       match "A", whereas a caseful version would. In a UTF mode, PCRE  always
       understands  the  concept  of case for characters whose values are less
       than 128, so caseless matching is always possible. For characters  with
       higher  values,  the  concept  of case is supported if PCRE is compiled
       with Unicode property support, but not otherwise.  If you want  to  use
       caseless  matching in a UTF mode for characters 128 and above, you must
       ensure that PCRE is compiled with Unicode property support as  well  as
       with UTF support.

       Characters  that  might  indicate  line breaks are never treated in any
       special way  when  matching  character  classes,  whatever  line-ending
       sequence  is  in  use,  and  whatever  setting  of  the PCRE_DOTALL and
       PCRE_MULTILINE options is used. A class such as [^a] always matches one
       of these characters.

       The  minus (hyphen) character can be used to specify a range of charac-
       ters in a character  class.  For  example,  [d-m]  matches  any  letter
       between  d  and  m,  inclusive.  If  a minus character is required in a
       class, it must be escaped with a backslash  or  appear  in  a  position
       where  it cannot be interpreted as indicating a range, typically as the
       first or last character in the class.

       It is not possible to have the literal character "]" as the end charac-
       ter  of a range. A pattern such as [W-]46] is interpreted as a class of
       two characters ("W" and "-") followed by a literal string "46]", so  it
       would  match  "W46]"  or  "-46]". However, if the "]" is escaped with a
       backslash it is interpreted as the end of range, so [W-\]46] is  inter-
       preted  as a class containing a range followed by two other characters.
       The octal or hexadecimal representation of "]" can also be used to  end
       a range.

       Ranges  operate in the collating sequence of character values. They can
       also  be  used  for  characters  specified  numerically,  for   example
       [\000-\037].  Ranges  can include any characters that are valid for the
       current mode.

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to [][\\^_`wxyzabc], matched caselessly, and  in  a  non-UTF  mode,  if
       character  tables  for  a French locale are in use, [\xc8-\xcb] matches
       accented E characters in both cases. In UTF modes,  PCRE  supports  the
       concept  of  case for characters with values greater than 128 only when
       it is compiled with Unicode property support.

       The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,  \V,
       \w, and \W may appear in a character class, and add the characters that
       they match to the class. For example, [\dABCDEF] matches any  hexadeci-
       mal  digit.  In  UTF modes, the PCRE_UCP option affects the meanings of
       \d, \s, \w and their upper case partners, just as  it  does  when  they
       appear  outside a character class, as described in the section entitled
       "Generic character types" above. The escape sequence \b has a different
       meaning  inside  a character class; it matches the backspace character.
       The sequences \B, \N, \R, and \X are not  special  inside  a  character
       class.  Like  any other unrecognized escape sequences, they are treated
       as the literal characters "B", "N", "R", and "X" by default, but  cause
       an error if the PCRE_EXTRA option is set.

       A  circumflex  can  conveniently  be used with the upper case character
       types to specify a more restricted set of characters than the  matching
       lower  case  type.  For example, the class [^\W_] matches any letter or
       digit, but not underscore, whereas [\w] includes underscore. A positive
       character class should be read as "something OR something OR ..." and a
       negative class as "NOT something AND NOT something AND NOT ...".

       The only metacharacters that are recognized in  character  classes  are
       backslash,  hyphen  (only  where  it can be interpreted as specifying a
       range), circumflex (only at the start), opening  square  bracket  (only
       when  it can be interpreted as introducing a POSIX class name - see the
       next section), and the terminating  closing  square  bracket.  However,
       escaping other non-alphanumeric characters does no harm.


POSIX CHARACTER CLASSES

       Perl supports the POSIX notation for character classes. This uses names
       enclosed by [: and :] within the enclosing square brackets.  PCRE  also
       supports this notation. For example,

         [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class
       names are:

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits and space
         space    white space (not quite the same as \s)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The "space" characters are HT (9), LF (10), VT (11), FF (12), CR  (13),
       and  space  (32). Notice that this list includes the VT character (code
       11). This makes "space" different to \s, which does not include VT (for
       Perl compatibility).

       The  name  "word"  is  a Perl extension, and "blank" is a GNU extension
       from Perl 5.8. Another Perl extension is negation, which  is  indicated
       by a ^ character after the colon. For example,

         [12[:^digit:]]

       matches  "1", "2", or any non-digit. PCRE (and Perl) also recognize the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

       By  default,  in  UTF modes, characters with values greater than 128 do
       not match any of the POSIX character classes. However, if the  PCRE_UCP
       option  is passed to pcre_compile(), some of the classes are changed so
       that Unicode character properties are used. This is achieved by replac-
       ing the POSIX classes by other sequences, as follows:

         [:alnum:]  becomes  \p{Xan}
         [:alpha:]  becomes  \p{L}
         [:blank:]  becomes  \h
         [:digit:]  becomes  \p{Nd}
         [:lower:]  becomes  \p{Ll}
         [:space:]  becomes  \p{Xps}
         [:upper:]  becomes  \p{Lu}
         [:word:]   becomes  \p{Xwd}

       Negated  versions,  such  as [:^alpha:] use \P instead of \p. The other
       POSIX classes are unchanged, and match only characters with code points
       less than 128.


VERTICAL BAR

       Vertical  bar characters are used to separate alternative patterns. For
       example, the pattern

         gilbert|sullivan

       matches either "gilbert" or "sullivan". Any number of alternatives  may
       appear,  and  an  empty  alternative  is  permitted (matching the empty
       string). The matching process tries each alternative in turn, from left
       to  right, and the first one that succeeds is used. If the alternatives
       are within a subpattern (defined below), "succeeds" means matching  the
       rest of the main pattern as well as the alternative in the subpattern.


INTERNAL OPTION SETTING

       The  settings  of  the  PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
       PCRE_EXTENDED options (which are Perl-compatible) can be  changed  from
       within  the  pattern  by  a  sequence  of  Perl option letters enclosed
       between "(?" and ")".  The option letters are

         i  for PCRE_CASELESS
         m  for PCRE_MULTILINE
         s  for PCRE_DOTALL
         x  for PCRE_EXTENDED

       For example, (?im) sets caseless, multiline matching. It is also possi-
       ble to unset these options by preceding the letter with a hyphen, and a
       combined setting and unsetting such as (?im-sx), which sets  PCRE_CASE-
       LESS  and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
       is also permitted. If a  letter  appears  both  before  and  after  the
       hyphen, the option is unset.

       The  PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
       can be changed in the same way as the Perl-compatible options by  using
       the characters J, U and X respectively.

       When  one  of  these  option  changes occurs at top level (that is, not
       inside subpattern parentheses), the change applies to the remainder  of
       the pattern that follows. If the change is placed right at the start of
       a pattern, PCRE extracts it into the global options (and it will there-
       fore show up in data extracted by the pcre_fullinfo() function).

       An  option  change  within a subpattern (see below for a description of
       subpatterns) affects only that part of the subpattern that follows  it,
       so

         (a(?i)b)c

       matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
       used).  By this means, options can be made to have  different  settings
       in  different parts of the pattern. Any changes made in one alternative
       do carry on into subsequent branches within the  same  subpattern.  For
       example,

         (a(?i)b|c)

       matches  "ab",  "aB",  "c",  and "C", even though when matching "C" the
       first branch is abandoned before the option setting.  This  is  because
       the  effects  of option settings happen at compile time. There would be
       some very weird behaviour otherwise.

       Note: There are other PCRE-specific options that  can  be  set  by  the
       application  when  the  compiling  or matching functions are called. In
       some cases the pattern can contain special leading  sequences  such  as
       (*CRLF)  to  override  what  the  application  has set or what has been
       defaulted.  Details  are  given  in  the  section   entitled   "Newline
       sequences"  above.  There  are  also  the (*UTF8), (*UTF16), and (*UCP)
       leading sequences that can be used to  set  UTF  and  Unicode  property
       modes;  they  are  equivalent to setting the PCRE_UTF8, PCRE_UTF16, and
       the PCRE_UCP options, respectively.


SUBPATTERNS

       Subpatterns are delimited by parentheses (round brackets), which can be
       nested.  Turning part of a pattern into a subpattern does two things:

       1. It localizes a set of alternatives. For example, the pattern

         cat(aract|erpillar|)

       matches  "cataract",  "caterpillar", or "cat". Without the parentheses,
       it would match "cataract", "erpillar" or an empty string.

       2. It sets up the subpattern as  a  capturing  subpattern.  This  means
       that,  when  the  whole  pattern  matches,  that portion of the subject
       string that matched the subpattern is passed back to the caller via the
       ovector  argument  of  the matching function. (This applies only to the
       traditional matching functions; the DFA matching functions do not  sup-
       port capturing.)

       Opening parentheses are counted from left to right (starting from 1) to
       obtain numbers for the  capturing  subpatterns.  For  example,  if  the
       string "the red king" is matched against the pattern

         the ((red|white) (king|queen))

       the captured substrings are "red king", "red", and "king", and are num-
       bered 1, 2, and 3, respectively.

       The fact that plain parentheses fulfil  two  functions  is  not  always
       helpful.   There are often times when a grouping subpattern is required
       without a capturing requirement. If an opening parenthesis is  followed
       by  a question mark and a colon, the subpattern does not do any captur-
       ing, and is not counted when computing the  number  of  any  subsequent
       capturing  subpatterns. For example, if the string "the white queen" is
       matched against the pattern

         the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and are numbered
       1 and 2. The maximum number of capturing subpatterns is 65535.

       As  a  convenient shorthand, if any option settings are required at the
       start of a non-capturing subpattern,  the  option  letters  may  appear
       between the "?" and the ":". Thus the two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are
       tried from left to right, and options are not reset until  the  end  of
       the  subpattern is reached, an option setting in one branch does affect
       subsequent branches, so the above patterns match "SUNDAY"  as  well  as
       "Saturday".


DUPLICATE SUBPATTERN NUMBERS

       Perl 5.10 introduced a feature whereby each alternative in a subpattern
       uses the same numbers for its capturing parentheses. Such a  subpattern
       starts  with (?| and is itself a non-capturing subpattern. For example,
       consider this pattern:

         (?|(Sat)ur|(Sun))day

       Because the two alternatives are inside a (?| group, both sets of  cap-
       turing  parentheses  are  numbered one. Thus, when the pattern matches,
       you can look at captured substring number  one,  whichever  alternative
       matched.  This  construct  is useful when you want to capture part, but
       not all, of one of a number of alternatives. Inside a (?| group, paren-
       theses  are  numbered as usual, but the number is reset at the start of
       each branch. The numbers of any capturing parentheses that  follow  the
       subpattern  start after the highest number used in any branch. The fol-
       lowing example is taken from the Perl documentation. The numbers under-
       neath show in which buffer the captured content will be stored.

         # before  ---------------branch-reset----------- after
         / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
         # 1            2         2  3        2     3     4

       A  back  reference  to a numbered subpattern uses the most recent value
       that is set for that number by any subpattern.  The  following  pattern
       matches "abcabc" or "defdef":

         /(?|(abc)|(def))\1/

       In  contrast,  a subroutine call to a numbered subpattern always refers
       to the first one in the pattern with the given  number.  The  following
       pattern matches "abcabc" or "defabc":

         /(?|(abc)|(def))(?1)/

       If  a condition test for a subpattern's having matched refers to a non-
       unique number, the test is true if any of the subpatterns of that  num-
       ber have matched.

       An  alternative approach to using this "branch reset" feature is to use
       duplicate named subpatterns, as described in the next section.


NAMED SUBPATTERNS

       Identifying capturing parentheses by number is simple, but  it  can  be
       very  hard  to keep track of the numbers in complicated regular expres-
       sions. Furthermore, if an  expression  is  modified,  the  numbers  may
       change.  To help with this difficulty, PCRE supports the naming of sub-
       patterns. This feature was not added to Perl until release 5.10. Python
       had  the  feature earlier, and PCRE introduced it at release 4.0, using
       the Python syntax. PCRE now supports both the Perl and the Python  syn-
       tax.  Perl  allows  identically  numbered subpatterns to have different
       names, but PCRE does not.

       In PCRE, a subpattern can be named in one of three  ways:  (?<name>...)
       or  (?'name'...)  as in Perl, or (?P<name>...) as in Python. References
       to capturing parentheses from other parts of the pattern, such as  back
       references,  recursion,  and conditions, can be made by name as well as
       by number.

       Names consist of up to  32  alphanumeric  characters  and  underscores.
       Named  capturing  parentheses  are  still  allocated numbers as well as
       names, exactly as if the names were not present. The PCRE API  provides
       function calls for extracting the name-to-number translation table from
       a compiled pattern. There is also a convenience function for extracting
       a captured substring by name.

       By  default, a name must be unique within a pattern, but it is possible
       to relax this constraint by setting the PCRE_DUPNAMES option at compile
       time.  (Duplicate  names are also always permitted for subpatterns with
       the same number, set up as described in the previous  section.)  Dupli-
       cate  names  can  be useful for patterns where only one instance of the
       named parentheses can match. Suppose you want to match the  name  of  a
       weekday,  either as a 3-letter abbreviation or as the full name, and in
       both cases you want to extract the abbreviation. This pattern (ignoring
       the line breaks) does the job:

         (?<DN>Mon|Fri|Sun)(?:day)?|
         (?<DN>Tue)(?:sday)?|
         (?<DN>Wed)(?:nesday)?|
         (?<DN>Thu)(?:rsday)?|
         (?<DN>Sat)(?:urday)?

       There  are  five capturing substrings, but only one is ever set after a
       match.  (An alternative way of solving this problem is to use a "branch
       reset" subpattern, as described in the previous section.)

       The  convenience  function  for extracting the data by name returns the
       substring for the first (and in this example, the only)  subpattern  of
       that  name  that  matched.  This saves searching to find which numbered
       subpattern it was.

       If you make a back reference to  a  non-unique  named  subpattern  from
       elsewhere  in the pattern, the one that corresponds to the first occur-
       rence of the name is used. In the absence of duplicate numbers (see the
       previous  section) this is the one with the lowest number. If you use a
       named reference in a condition test (see the section  about  conditions
       below),  either  to check whether a subpattern has matched, or to check
       for recursion, all subpatterns with the same name are  tested.  If  the
       condition  is  true for any one of them, the overall condition is true.
       This is the same behaviour as testing by number. For further details of
       the interfaces for handling named subpatterns, see the pcreapi documen-
       tation.

       Warning: You cannot use different names to distinguish between two sub-
       patterns  with  the same number because PCRE uses only the numbers when
       matching. For this reason, an error is given at compile time if differ-
       ent  names  are given to subpatterns with the same number. However, you
       can give the same name to subpatterns with the same number,  even  when
       PCRE_DUPNAMES is not set.


REPETITION

       Repetition  is  specified  by  quantifiers, which can follow any of the
       following items:

         a literal data character
         the dot metacharacter
         the \C escape sequence
         the \X escape sequence
         the \R escape sequence
         an escape such as \d or \pL that matches a single character
         a character class
         a back reference (see next section)
         a parenthesized subpattern (including assertions)
         a subroutine call to a subpattern (recursive or otherwise)

       The general repetition quantifier specifies a minimum and maximum  num-
       ber  of  permitted matches, by giving the two numbers in curly brackets
       (braces), separated by a comma. The numbers must be  less  than  65536,
       and the first must be less than or equal to the second. For example:

         z{2,4}

       matches  "zz",  "zzz",  or  "zzzz". A closing brace on its own is not a
       special character. If the second number is omitted, but  the  comma  is
       present,  there  is  no upper limit; if the second number and the comma
       are both omitted, the quantifier specifies an exact number of  required
       matches. Thus

         [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, while

         \d{8}

       matches  exactly  8  digits. An opening curly bracket that appears in a
       position where a quantifier is not allowed, or one that does not  match
       the  syntax of a quantifier, is taken as a literal character. For exam-
       ple, {,6} is not a quantifier, but a literal string of four characters.

       In UTF modes, quantifiers apply to characters rather than to individual
       data  units. Thus, for example, \x{100}{2} matches two characters, each
       of which is represented by a two-byte sequence in a UTF-8 string. Simi-
       larly,  \X{3}  matches  three Unicode extended sequences, each of which
       may be several data units long (and they may be of different lengths).

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be use-
       ful for subpatterns that are referenced as subroutines  from  elsewhere
       in the pattern (but see also the section entitled "Defining subpatterns
       for use by reference only" below). Items other  than  subpatterns  that
       have a {0} quantifier are omitted from the compiled pattern.

       For  convenience, the three most common quantifiers have single-charac-
       ter abbreviations:

         *    is equivalent to {0,}
         +    is equivalent to {1,}
         ?    is equivalent to {0,1}

       It is possible to construct infinite loops by  following  a  subpattern
       that can match no characters with a quantifier that has no upper limit,
       for example:

         (a?)*

       Earlier versions of Perl and PCRE used to give an error at compile time
       for  such  patterns. However, because there are cases where this can be
       useful, such patterns are now accepted, but if any  repetition  of  the
       subpattern  does in fact match no characters, the loop is forcibly bro-
       ken.

       By default, the quantifiers are "greedy", that is, they match  as  much
       as  possible  (up  to  the  maximum number of permitted times), without
       causing the rest of the pattern to fail. The classic example  of  where
       this gives problems is in trying to match comments in C programs. These
       appear between /* and */ and within the comment,  individual  *  and  /
       characters  may  appear. An attempt to match C comments by applying the
       pattern

         /\*.*\*/

       to the string

         /* first comment */  not comment  /* second comment */

       fails, because it matches the entire string owing to the greediness  of
       the .*  item.

       However,  if  a quantifier is followed by a question mark, it ceases to
       be greedy, and instead matches the minimum number of times possible, so
       the pattern

         /\*.*?\*/

       does  the  right  thing with the C comments. The meaning of the various
       quantifiers is not otherwise changed,  just  the  preferred  number  of
       matches.   Do  not  confuse this use of question mark with its use as a
       quantifier in its own right. Because it has two uses, it can  sometimes
       appear doubled, as in

         \d??\d

       which matches one digit by preference, but can match two if that is the
       only way the rest of the pattern matches.

       If the PCRE_UNGREEDY option is set (an option that is not available  in
       Perl),  the  quantifiers are not greedy by default, but individual ones
       can be made greedy by following them with a  question  mark.  In  other
       words, it inverts the default behaviour.

       When  a  parenthesized  subpattern  is quantified with a minimum repeat
       count that is greater than 1 or with a limited maximum, more memory  is
       required  for  the  compiled  pattern, in proportion to the size of the
       minimum or maximum.

       If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
       alent  to  Perl's  /s) is set, thus allowing the dot to match newlines,
       the pattern is implicitly anchored, because whatever  follows  will  be
       tried  against every character position in the subject string, so there
       is no point in retrying the overall match at  any  position  after  the
       first.  PCRE  normally treats such a pattern as though it were preceded
       by \A.

       In cases where it is known that the subject  string  contains  no  new-
       lines,  it  is  worth setting PCRE_DOTALL in order to obtain this opti-
       mization, or alternatively using ^ to indicate anchoring explicitly.

       However, there is one situation where the optimization cannot be  used.
       When .*  is inside capturing parentheses that are the subject of a back
       reference elsewhere in the pattern, a match at the start may fail where
       a later one succeeds. Consider, for example:

         (.*)abc\1

       If  the subject is "xyz123abc123" the match point is the fourth charac-
       ter. For this reason, such a pattern is not implicitly anchored.

       When a capturing subpattern is repeated, the value captured is the sub-
       string that matched the final iteration. For example, after

         (tweedle[dume]{3}\s*)+

       has matched "tweedledum tweedledee" the value of the captured substring
       is "tweedledee". However, if there are  nested  capturing  subpatterns,
       the  corresponding captured values may have been set in previous itera-
       tions. For example, after

         /(a|(b))+/

       matches "aba" the value of the second captured substring is "b".


ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

       With both maximizing ("greedy") and minimizing ("ungreedy"  or  "lazy")
       repetition,  failure  of what follows normally causes the repeated item
       to be re-evaluated to see if a different number of repeats  allows  the
       rest  of  the pattern to match. Sometimes it is useful to prevent this,
       either to change the nature of the match, or to cause it  fail  earlier
       than  it otherwise might, when the author of the pattern knows there is
       no point in carrying on.

       Consider, for example, the pattern \d+foo when applied to  the  subject
       line

         123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       action of the matcher is to try again with only 5 digits  matching  the
       \d+  item,  and  then  with  4,  and  so on, before ultimately failing.
       "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides
       the  means for specifying that once a subpattern has matched, it is not
       to be re-evaluated in this way.

       If we use atomic grouping for the previous example, the  matcher  gives
       up  immediately  on failing to match "foo" the first time. The notation
       is a kind of special parenthesis, starting with (?> as in this example:

         (?>\d+)foo

       This kind of parenthesis "locks up" the  part of the  pattern  it  con-
       tains  once  it  has matched, and a failure further into the pattern is
       prevented from backtracking into it. Backtracking past it  to  previous
       items, however, works as normal.

       An  alternative  description  is that a subpattern of this type matches
       the string of characters that an  identical  standalone  pattern  would
       match, if anchored at the current point in the subject string.

       Atomic grouping subpatterns are not capturing subpatterns. Simple cases
       such as the above example can be thought of as a maximizing repeat that
       must  swallow  everything  it can. So, while both \d+ and \d+? are pre-
       pared to adjust the number of digits they match in order  to  make  the
       rest of the pattern match, (?>\d+) can only match an entire sequence of
       digits.

       Atomic groups in general can of course contain arbitrarily  complicated
       subpatterns,  and  can  be  nested. However, when the subpattern for an
       atomic group is just a single repeated item, as in the example above, a
       simpler  notation,  called  a "possessive quantifier" can be used. This
       consists of an additional + character  following  a  quantifier.  Using
       this notation, the previous example can be rewritten as

         \d++foo

       Note that a possessive quantifier can be used with an entire group, for
       example:

         (abc|xyz){2,3}+

       Possessive  quantifiers  are  always  greedy;  the   setting   of   the
       PCRE_UNGREEDY option is ignored. They are a convenient notation for the
       simpler forms of atomic group. However, there is no difference  in  the
       meaning  of  a  possessive  quantifier and the equivalent atomic group,
       though there may be a performance  difference;  possessive  quantifiers
       should be slightly faster.

       The  possessive  quantifier syntax is an extension to the Perl 5.8 syn-
       tax.  Jeffrey Friedl originated the idea (and the name)  in  the  first
       edition of his book. Mike McCloskey liked it, so implemented it when he
       built Sun's Java package, and PCRE copied it from there. It  ultimately
       found its way into Perl at release 5.10.

       PCRE has an optimization that automatically "possessifies" certain sim-
       ple pattern constructs. For example, the sequence  A+B  is  treated  as
       A++B  because  there is no point in backtracking into a sequence of A's
       when B must follow.

       When a pattern contains an unlimited repeat inside  a  subpattern  that
       can  itself  be  repeated  an  unlimited number of times, the use of an
       atomic group is the only way to avoid some  failing  matches  taking  a
       very long time indeed. The pattern

         (\D+|<\d+>)*[!?]

       matches  an  unlimited number of substrings that either consist of non-
       digits, or digits enclosed in <>, followed by either ! or  ?.  When  it
       matches, it runs quickly. However, if it is applied to

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it  takes  a  long  time  before reporting failure. This is because the
       string can be divided between the internal \D+ repeat and the  external
       *  repeat  in  a  large  number of ways, and all have to be tried. (The
       example uses [!?] rather than a single character at  the  end,  because
       both  PCRE  and  Perl have an optimization that allows for fast failure
       when a single character is used. They remember the last single  charac-
       ter  that  is required for a match, and fail early if it is not present
       in the string.) If the pattern is changed so that  it  uses  an  atomic
       group, like this:

         ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens quickly.


BACK REFERENCES

       Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a back reference to a capturing sub-
       pattern  earlier  (that is, to its left) in the pattern, provided there
       have been that many previous capturing left parentheses.

       However, if the decimal number following the backslash is less than 10,
       it  is  always  taken  as a back reference, and causes an error only if
       there are not that many capturing left parentheses in the  entire  pat-
       tern.  In  other words, the parentheses that are referenced need not be
       to the left of the reference for numbers less than 10. A "forward  back
       reference"  of  this  type can make sense when a repetition is involved
       and the subpattern to the right has participated in an  earlier  itera-
       tion.

       It  is  not  possible to have a numerical "forward back reference" to a
       subpattern whose number is 10 or  more  using  this  syntax  because  a
       sequence  such  as  \50 is interpreted as a character defined in octal.
       See the subsection entitled "Non-printing characters" above for further
       details  of  the  handling of digits following a backslash. There is no
       such problem when named parentheses are used. A back reference  to  any
       subpattern is possible using named parentheses (see below).

       Another  way  of  avoiding  the ambiguity inherent in the use of digits
       following a backslash is to use the \g  escape  sequence.  This  escape
       must be followed by an unsigned number or a negative number, optionally
       enclosed in braces. These examples are all identical:

         (ring), \1
         (ring), \g1
         (ring), \g{1}

       An unsigned number specifies an absolute reference without the  ambigu-
       ity that is present in the older syntax. It is also useful when literal
       digits follow the reference. A negative number is a relative reference.
       Consider this example:

         (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a reference to the most recently started captur-
       ing subpattern before \g, that is, is it equivalent to \2 in this exam-
       ple.   Similarly, \g{-2} would be equivalent to \1. The use of relative
       references can be helpful in long patterns, and also in  patterns  that
       are  created  by  joining  together  fragments  that contain references
       within themselves.

       A back reference matches whatever actually matched the  capturing  sub-
       pattern  in  the  current subject string, rather than anything matching
       the subpattern itself (see "Subpatterns as subroutines" below for a way
       of doing that). So the pattern

         (sens|respons)e and \1ibility

       matches  "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If caseful matching is in force at  the
       time  of the back reference, the case of letters is relevant. For exam-
       ple,

         ((?i)rah)\s+\1

       matches "rah rah" and "RAH RAH", but not "RAH  rah",  even  though  the
       original capturing subpattern is matched caselessly.

       There  are  several  different ways of writing back references to named
       subpatterns. The .NET syntax \k{name} and the Perl syntax  \k<name>  or
       \k'name'  are supported, as is the Python syntax (?P=name). Perl 5.10's
       unified back reference syntax, in which \g can be used for both numeric
       and  named  references,  is  also supported. We could rewrite the above
       example in any of the following ways:

         (?<p1>(?i)rah)\s+\k<p1>
         (?'p1'(?i)rah)\s+\k{p1}
         (?P<p1>(?i)rah)\s+(?P=p1)
         (?<p1>(?i)rah)\s+\g{p1}

       A subpattern that is referenced by  name  may  appear  in  the  pattern
       before or after the reference.

       There  may be more than one back reference to the same subpattern. If a
       subpattern has not actually been used in a particular match,  any  back
       references to it always fail by default. For example, the pattern

         (a|(bc))\2

       always  fails  if  it starts to match "a" rather than "bc". However, if
       the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
       ence to an unset value matches an empty string.

       Because  there may be many capturing parentheses in a pattern, all dig-
       its following a backslash are taken as part of a potential back  refer-
       ence  number.   If  the  pattern continues with a digit character, some
       delimiter must  be  used  to  terminate  the  back  reference.  If  the
       PCRE_EXTENDED  option  is  set, this can be white space. Otherwise, the
       \g{ syntax or an empty comment (see "Comments" below) can be used.

   Recursive back references

       A back reference that occurs inside the parentheses to which it  refers
       fails  when  the subpattern is first used, so, for example, (a\1) never
       matches.  However, such references can be useful inside  repeated  sub-
       patterns. For example, the pattern

         (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
       ation of the subpattern,  the  back  reference  matches  the  character
       string  corresponding  to  the previous iteration. In order for this to
       work, the pattern must be such that the first iteration does  not  need
       to  match the back reference. This can be done using alternation, as in
       the example above, or by a quantifier with a minimum of zero.

       Back references of this type cause the group that they reference to  be
       treated  as  an atomic group.  Once the whole group has been matched, a
       subsequent matching failure cannot cause backtracking into  the  middle
       of the group.


ASSERTIONS

       An  assertion  is  a  test on the characters following or preceding the
       current matching point that does not actually consume  any  characters.
       The  simple  assertions  coded  as  \b, \B, \A, \G, \Z, \z, ^ and $ are
       described above.

       More complicated assertions are coded as  subpatterns.  There  are  two
       kinds:  those  that  look  ahead of the current position in the subject
       string, and those that look  behind  it.  An  assertion  subpattern  is
       matched  in  the  normal way, except that it does not cause the current
       matching position to be changed.

       Assertion subpatterns are not capturing subpatterns. If such an  asser-
       tion  contains  capturing  subpatterns within it, these are counted for
       the purposes of numbering the capturing subpatterns in the  whole  pat-
       tern.  However,  substring  capturing  is carried out only for positive
       assertions, because it does not make sense for negative assertions.

       For compatibility with Perl, assertion  subpatterns  may  be  repeated;
       though  it  makes  no sense to assert the same thing several times, the
       side effect of capturing parentheses may  occasionally  be  useful.  In
       practice, there only three cases:

       (1)  If  the  quantifier  is  {0}, the assertion is never obeyed during
       matching.  However, it may  contain  internal  capturing  parenthesized
       groups that are called from elsewhere via the subroutine mechanism.

       (2)  If quantifier is {0,n} where n is greater than zero, it is treated
       as if it were {0,1}. At run time, the rest  of  the  pattern  match  is
       tried with and without the assertion, the order depending on the greed-
       iness of the quantifier.

       (3) If the minimum repetition is greater than zero, the  quantifier  is
       ignored.   The  assertion  is  obeyed just once when encountered during
       matching.

   Lookahead assertions

       Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

         \w+(?=;)

       matches  a word followed by a semicolon, but does not include the semi-
       colon in the match, and

         foo(?!bar)

       matches any occurrence of "foo" that is not  followed  by  "bar".  Note
       that the apparently similar pattern

         (?!foo)bar

       does  not  find  an  occurrence  of "bar" that is preceded by something
       other than "foo"; it finds any occurrence of "bar" whatsoever,  because
       the assertion (?!foo) is always true when the next three characters are
       "bar". A lookbehind assertion is needed to achieve the other effect.

       If you want to force a matching failure at some point in a pattern, the
       most  convenient  way  to  do  it  is with (?!) because an empty string
       always matches, so an assertion that requires there not to be an  empty
       string must always fail.  The backtracking control verb (*FAIL) or (*F)
       is a synonym for (?!).

   Lookbehind assertions

       Lookbehind assertions start with (?<= for positive assertions and  (?<!
       for negative assertions. For example,

         (?<!foo)bar

       does  find  an  occurrence  of "bar" that is not preceded by "foo". The
       contents of a lookbehind assertion are restricted  such  that  all  the
       strings it matches must have a fixed length. However, if there are sev-
       eral top-level alternatives, they do not all  have  to  have  the  same
       fixed length. Thus

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

       causes  an  error at compile time. Branches that match different length
       strings are permitted only at the top level of a lookbehind  assertion.
       This is an extension compared with Perl, which requires all branches to
       match the same length of string. An assertion such as

         (?<=ab(c|de))

       is not permitted, because its single top-level  branch  can  match  two
       different lengths, but it is acceptable to PCRE if rewritten to use two
       top-level branches:

         (?<=abc|abde)

       In some cases, the escape sequence \K (see above) can be  used  instead
       of a lookbehind assertion to get round the fixed-length restriction.

       The  implementation  of lookbehind assertions is, for each alternative,
       to temporarily move the current position back by the fixed  length  and
       then try to match. If there are insufficient characters before the cur-
       rent position, the assertion fails.

       In a UTF mode, PCRE does not allow the \C escape (which matches a  sin-
       gle  data  unit even in a UTF mode) to appear in lookbehind assertions,
       because it makes it impossible to calculate the length of  the  lookbe-
       hind.  The \X and \R escapes, which can match different numbers of data
       units, are also not permitted.

       "Subroutine" calls (see below) such as (?2) or (?&X) are  permitted  in
       lookbehinds,  as  long as the subpattern matches a fixed-length string.
       Recursion, however, is not supported.

       Possessive quantifiers can  be  used  in  conjunction  with  lookbehind
       assertions to specify efficient matching of fixed-length strings at the
       end of subject strings. Consider a simple pattern such as

         abcd$

       when applied to a long string that does  not  match.  Because  matching
       proceeds from left to right, PCRE will look for each "a" in the subject
       and then see if what follows matches the rest of the  pattern.  If  the
       pattern is specified as

         ^.*abcd$

       the  initial .* matches the entire string at first, but when this fails
       (because there is no following "a"), it backtracks to match all but the
       last  character,  then all but the last two characters, and so on. Once
       again the search for "a" covers the entire string, from right to  left,
       so we are no better off. However, if the pattern is written as

         ^.*+(?<=abcd)

       there  can  be  no backtracking for the .*+ item; it can match only the
       entire string. The subsequent lookbehind assertion does a  single  test
       on  the last four characters. If it fails, the match fails immediately.
       For long strings, this approach makes a significant difference  to  the
       processing time.

   Using multiple assertions

       Several assertions (of any sort) may occur in succession. For example,

         (?<=\d{3})(?<!999)foo

       matches  "foo" preceded by three digits that are not "999". Notice that
       each of the assertions is applied independently at the  same  point  in
       the  subject  string.  First  there  is a check that the previous three
       characters are all digits, and then there is  a  check  that  the  same
       three characters are not "999".  This pattern does not match "foo" pre-
       ceded by six characters, the first of which are  digits  and  the  last
       three  of  which  are not "999". For example, it doesn't match "123abc-
       foo". A pattern to do that is

         (?<=\d{3}...)(?<!999)foo

       This time the first assertion looks at the  preceding  six  characters,
       checking that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

       Assertions can be nested in any combination. For example,

         (?<=(?<!foo)bar)baz

       matches an occurrence of "baz" that is preceded by "bar" which in  turn
       is not preceded by "foo", while

         (?<=\d{3}(?!999)...)foo

       is  another pattern that matches "foo" preceded by three digits and any
       three characters that are not "999".


CONDITIONAL SUBPATTERNS

       It is possible to cause the matching process to obey a subpattern  con-
       ditionally  or to choose between two alternative subpatterns, depending
       on the result of an assertion, or whether a specific capturing  subpat-
       tern  has  already  been matched. The two possible forms of conditional
       subpattern are:

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

       If the condition is satisfied, the yes-pattern is used;  otherwise  the
       no-pattern  (if  present)  is used. If there are more than two alterna-
       tives in the subpattern, a compile-time error occurs. Each of  the  two
       alternatives may itself contain nested subpatterns of any form, includ-
       ing  conditional  subpatterns;  the  restriction  to  two  alternatives
       applies only at the level of the condition. This pattern fragment is an
       example where the alternatives are complex:

         (?(1) (A|B|C) | (D | (?(2)E|F) | E) )


       There are four kinds of condition: references  to  subpatterns,  refer-
       ences to recursion, a pseudo-condition called DEFINE, and assertions.

   Checking for a used subpattern by number

       If  the  text between the parentheses consists of a sequence of digits,
       the condition is true if a capturing subpattern of that number has pre-
       viously  matched.  If  there is more than one capturing subpattern with
       the same number (see the earlier  section  about  duplicate  subpattern
       numbers),  the condition is true if any of them have matched. An alter-
       native notation is to precede the digits with a plus or minus sign.  In
       this  case, the subpattern number is relative rather than absolute. The
       most recently opened parentheses can be referenced by (?(-1), the  next
       most  recent  by (?(-2), and so on. Inside loops it can also make sense
       to refer to subsequent groups. The next parentheses to be opened can be
       referenced  as (?(+1), and so on. (The value zero in any of these forms
       is not used; it provokes a compile-time error.)

       Consider the following pattern, which  contains  non-significant  white
       space to make it more readable (assume the PCRE_EXTENDED option) and to
       divide it into three parts for ease of discussion:

         ( \( )?    [^()]+    (?(1) \) )

       The first part matches an optional opening  parenthesis,  and  if  that
       character is present, sets it as the first captured substring. The sec-
       ond part matches one or more characters that are not  parentheses.  The
       third  part  is  a conditional subpattern that tests whether or not the
       first set of parentheses matched. If they  did,  that  is,  if  subject
       started  with an opening parenthesis, the condition is true, and so the
       yes-pattern is executed and a closing parenthesis is  required.  Other-
       wise,  since no-pattern is not present, the subpattern matches nothing.
       In other words, this pattern matches  a  sequence  of  non-parentheses,
       optionally enclosed in parentheses.

       If  you  were  embedding  this pattern in a larger one, you could use a
       relative reference:

         ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

       This makes the fragment independent of the parentheses  in  the  larger
       pattern.

   Checking for a used subpattern by name

       Perl  uses  the  syntax  (?(<name>)...) or (?('name')...) to test for a
       used subpattern by name. For compatibility  with  earlier  versions  of
       PCRE,  which  had this facility before Perl, the syntax (?(name)...) is
       also recognized. However, there is a possible ambiguity with this  syn-
       tax,  because  subpattern  names  may  consist entirely of digits. PCRE
       looks first for a named subpattern; if it cannot find one and the  name
       consists  entirely  of digits, PCRE looks for a subpattern of that num-
       ber, which must be greater than zero. Using subpattern names that  con-
       sist entirely of digits is not recommended.

       Rewriting the above example to use a named subpattern gives this:

         (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )

       If  the  name used in a condition of this kind is a duplicate, the test
       is applied to all subpatterns of the same name, and is true if any  one
       of them has matched.

   Checking for pattern recursion

       If the condition is the string (R), and there is no subpattern with the
       name R, the condition is true if a recursive call to the whole  pattern
       or any subpattern has been made. If digits or a name preceded by amper-
       sand follow the letter R, for example:

         (?(R3)...) or (?(R&name)...)

       the condition is true if the most recent recursion is into a subpattern
       whose number or name is given. This condition does not check the entire
       recursion stack. If the name used in a condition  of  this  kind  is  a
       duplicate, the test is applied to all subpatterns of the same name, and
       is true if any one of them is the most recent recursion.

       At "top level", all these recursion test  conditions  are  false.   The
       syntax for recursive patterns is described below.

   Defining subpatterns for use by reference only

       If  the  condition  is  the string (DEFINE), and there is no subpattern
       with the name DEFINE, the condition is  always  false.  In  this  case,
       there  may  be  only  one  alternative  in the subpattern. It is always
       skipped if control reaches this point  in  the  pattern;  the  idea  of
       DEFINE  is that it can be used to define subroutines that can be refer-
       enced from elsewhere. (The use of subroutines is described below.)  For
       example,  a  pattern  to match an IPv4 address such as "192.168.23.245"
       could be written like this (ignore white space and line breaks):

         (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
         \b (?&byte) (\.(?&byte)){3} \b

       The first part of the pattern is a DEFINE group inside which a  another
       group  named "byte" is defined. This matches an individual component of
       an IPv4 address (a number less than 256). When  matching  takes  place,
       this  part  of  the pattern is skipped because DEFINE acts like a false
       condition. The rest of the pattern uses references to the  named  group
       to  match the four dot-separated components of an IPv4 address, insist-
       ing on a word boundary at each end.

   Assertion conditions

       If the condition is not in any of the above  formats,  it  must  be  an
       assertion.   This may be a positive or negative lookahead or lookbehind
       assertion. Consider  this  pattern,  again  containing  non-significant
       white space, and with the two alternatives on the second line:

         (?(?=[^a-z]*[a-z])
         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The  condition  is  a  positive  lookahead  assertion  that  matches an
       optional sequence of non-letters followed by a letter. In other  words,
       it  tests  for the presence of at least one letter in the subject. If a
       letter is found, the subject is matched against the first  alternative;
       otherwise  it  is  matched  against  the  second.  This pattern matches
       strings in one of the two forms dd-aaa-dd or dd-dd-dd,  where  aaa  are
       letters and dd are digits.


COMMENTS

       There are two ways of including comments in patterns that are processed
       by PCRE. In both cases, the start of the comment must not be in a char-
       acter class, nor in the middle of any other sequence of related charac-
       ters such as (?: or a subpattern name or number.  The  characters  that
       make up a comment play no part in the pattern matching.

       The  sequence (?# marks the start of a comment that continues up to the
       next closing parenthesis. Nested parentheses are not permitted. If  the
       PCRE_EXTENDED option is set, an unescaped # character also introduces a
       comment, which in this case continues to  immediately  after  the  next
       newline  character  or character sequence in the pattern. Which charac-
       ters are interpreted as newlines is controlled by the options passed to
       a  compiling function or by a special sequence at the start of the pat-
       tern, as described in the section entitled "Newline conventions" above.
       Note that the end of this type of comment is a literal newline sequence
       in the pattern; escape sequences that happen to represent a newline  do
       not  count.  For  example,  consider this pattern when PCRE_EXTENDED is
       set, and the default newline convention is in force:

         abc #comment \n still comment

       On encountering the # character, pcre_compile()  skips  along,  looking
       for  a newline in the pattern. The sequence \n is still literal at this
       stage, so it does not terminate the comment. Only an  actual  character
       with the code value 0x0a (the default newline) does so.


RECURSIVE PATTERNS

       Consider  the problem of matching a string in parentheses, allowing for
       unlimited nested parentheses. Without the use of  recursion,  the  best
       that  can  be  done  is  to use a pattern that matches up to some fixed
       depth of nesting. It is not possible to  handle  an  arbitrary  nesting
       depth.

       For some time, Perl has provided a facility that allows regular expres-
       sions to recurse (amongst other things). It does this by  interpolating
       Perl  code in the expression at run time, and the code can refer to the
       expression itself. A Perl pattern using code interpolation to solve the
       parentheses problem can be created like this:

         $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears.

       Obviously, PCRE cannot support the interpolation of Perl code. Instead,
       it  supports  special  syntax  for recursion of the entire pattern, and
       also for individual subpattern recursion.  After  its  introduction  in
       PCRE  and  Python,  this  kind of recursion was subsequently introduced
       into Perl at release 5.10.

       A special item that consists of (? followed by a  number  greater  than
       zero  and  a  closing parenthesis is a recursive subroutine call of the
       subpattern of the given number, provided that  it  occurs  inside  that
       subpattern.  (If  not,  it is a non-recursive subroutine call, which is
       described in the next section.) The special item  (?R)  or  (?0)  is  a
       recursive call of the entire regular expression.

       This  PCRE  pattern  solves  the nested parentheses problem (assume the
       PCRE_EXTENDED option is set so that white space is ignored):

         \( ( [^()]++ | (?R) )* \)

       First it matches an opening parenthesis. Then it matches any number  of
       substrings  which  can  either  be  a sequence of non-parentheses, or a
       recursive match of the pattern itself (that is, a  correctly  parenthe-
       sized substring).  Finally there is a closing parenthesis. Note the use
       of a possessive quantifier to avoid backtracking into sequences of non-
       parentheses.

       If  this  were  part of a larger pattern, you would not want to recurse
       the entire pattern, so instead you could use this:

         ( \( ( [^()]++ | (?1) )* \) )

       We have put the pattern into parentheses, and caused the  recursion  to
       refer to them instead of the whole pattern.

       In  a  larger  pattern,  keeping  track  of  parenthesis numbers can be
       tricky. This is made easier by the use of relative references.  Instead
       of (?1) in the pattern above you can write (?-2) to refer to the second
       most recently opened parentheses  preceding  the  recursion.  In  other
       words,  a  negative  number counts capturing parentheses leftwards from
       the point at which it is encountered.

       It is also possible to refer to  subsequently  opened  parentheses,  by
       writing  references  such  as (?+2). However, these cannot be recursive
       because the reference is not inside the  parentheses  that  are  refer-
       enced.  They are always non-recursive subroutine calls, as described in
       the next section.

       An alternative approach is to use named parentheses instead.  The  Perl
       syntax  for  this  is (?&name); PCRE's earlier syntax (?P>name) is also
       supported. We could rewrite the above example as follows:

         (?<pn> \( ( [^()]++ | (?&pn) )* \) )

       If there is more than one subpattern with the same name,  the  earliest
       one is used.

       This  particular  example pattern that we have been looking at contains
       nested unlimited repeats, and so the use of a possessive quantifier for
       matching strings of non-parentheses is important when applying the pat-
       tern to strings that do not match. For example, when  this  pattern  is
       applied to

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it  yields  "no  match" quickly. However, if a possessive quantifier is
       not used, the match runs for a very long time indeed because there  are
       so  many  different  ways the + and * repeats can carve up the subject,
       and all have to be tested before failure can be reported.

       At the end of a match, the values of capturing  parentheses  are  those
       from  the outermost level. If you want to obtain intermediate values, a
       callout function can be used (see below and the pcrecallout  documenta-
       tion). If the pattern above is matched against

         (ab(cd)ef)

       the  value  for  the  inner capturing parentheses (numbered 2) is "ef",
       which is the last value taken on at the top level. If a capturing  sub-
       pattern  is  not  matched at the top level, its final captured value is
       unset, even if it was (temporarily) set at a deeper  level  during  the
       matching process.

       If  there are more than 15 capturing parentheses in a pattern, PCRE has
       to obtain extra memory to store data during a recursion, which it  does
       by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
       can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.

       Do not confuse the (?R) item with the condition (R),  which  tests  for
       recursion.   Consider  this pattern, which matches text in angle brack-
       ets, allowing for arbitrary nesting. Only digits are allowed in  nested
       brackets  (that is, when recursing), whereas any characters are permit-
       ted at the outer level.

         < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

       In this pattern, (?(R) is the start of a conditional  subpattern,  with
       two  different  alternatives for the recursive and non-recursive cases.
       The (?R) item is the actual recursive call.

   Differences in recursion processing between PCRE and Perl

       Recursion processing in PCRE differs from Perl in two  important  ways.
       In  PCRE (like Python, but unlike Perl), a recursive subpattern call is
       always treated as an atomic group. That is, once it has matched some of
       the subject string, it is never re-entered, even if it contains untried
       alternatives and there is a subsequent matching failure.  This  can  be
       illustrated  by the following pattern, which purports to match a palin-
       dromic string that contains an odd number of characters  (for  example,
       "a", "aba", "abcba", "abcdcba"):

         ^(.|(.)(?1)\2)$

       The idea is that it either matches a single character, or two identical
       characters surrounding a sub-palindrome. In Perl, this  pattern  works;
       in  PCRE  it  does  not if the pattern is longer than three characters.
       Consider the subject string "abcba":

       At the top level, the first character is matched, but as it is  not  at
       the end of the string, the first alternative fails; the second alterna-
       tive is taken and the recursion kicks in. The recursive call to subpat-
       tern  1  successfully  matches the next character ("b"). (Note that the
       beginning and end of line tests are not part of the recursion).

       Back at the top level, the next character ("c") is compared  with  what
       subpattern  2 matched, which was "a". This fails. Because the recursion
       is treated as an atomic group, there are now  no  backtracking  points,
       and  so  the  entire  match fails. (Perl is able, at this point, to re-
       enter the recursion and try the second alternative.)  However,  if  the
       pattern is written with the alternatives in the other order, things are
       different:

         ^((.)(?1)\2|.)$

       This time, the recursing alternative is tried first, and  continues  to
       recurse  until  it runs out of characters, at which point the recursion
       fails. But this time we do have  another  alternative  to  try  at  the
       higher  level.  That  is  the  big difference: in the previous case the
       remaining alternative is at a deeper recursion level, which PCRE cannot
       use.

       To  change  the pattern so that it matches all palindromic strings, not
       just those with an odd number of characters, it is tempting  to  change
       the pattern to this:

         ^((.)(?1)\2|.?)$

       Again,  this  works  in Perl, but not in PCRE, and for the same reason.
       When a deeper recursion has matched a single character,  it  cannot  be
       entered  again  in  order  to match an empty string. The solution is to
       separate the two cases, and write out the odd and even cases as  alter-
       natives at the higher level:

         ^(?:((.)(?1)\2|)|((.)(?3)\4|.))

       If  you  want  to match typical palindromic phrases, the pattern has to
       ignore all non-word characters, which can be done like this:

         ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$

       If run with the PCRE_CASELESS option, this pattern matches phrases such
       as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
       Perl. Note the use of the possessive quantifier *+ to avoid  backtrack-
       ing  into  sequences of non-word characters. Without this, PCRE takes a
       great deal longer (ten times or more) to  match  typical  phrases,  and
       Perl takes so long that you think it has gone into a loop.

       WARNING:  The  palindrome-matching patterns above work only if the sub-
       ject string does not start with a palindrome that is shorter  than  the
       entire  string.  For example, although "abcba" is correctly matched, if
       the subject is "ababa", PCRE finds the palindrome "aba" at  the  start,
       then  fails at top level because the end of the string does not follow.
       Once again, it cannot jump back into the recursion to try other  alter-
       natives, so the entire match fails.

       The  second  way  in which PCRE and Perl differ in their recursion pro-
       cessing is in the handling of captured values. In Perl, when a  subpat-
       tern  is  called recursively or as a subpattern (see the next section),
       it has no access to any values that were captured  outside  the  recur-
       sion,  whereas  in  PCRE  these values can be referenced. Consider this
       pattern:

         ^(.)(\1|a(?2))

       In PCRE, this pattern matches "bab". The  first  capturing  parentheses
       match  "b",  then in the second group, when the back reference \1 fails
       to match "b", the second alternative matches "a" and then recurses.  In
       the  recursion,  \1 does now match "b" and so the whole match succeeds.
       In Perl, the pattern fails to match because inside the  recursive  call
       \1 cannot access the externally set value.


SUBPATTERNS AS SUBROUTINES

       If  the  syntax for a recursive subpattern call (either by number or by
       name) is used outside the parentheses to which it refers,  it  operates
       like  a subroutine in a programming language. The called subpattern may
       be defined before or after the reference. A numbered reference  can  be
       absolute or relative, as in these examples:

         (...(absolute)...)...(?2)...
         (...(relative)...)...(?-1)...
         (...(?+1)...(relative)...

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

       matches  "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If instead the pattern

         (sens|respons)e and (?1)ibility

       is used, it does match "sense and responsibility" as well as the  other
       two  strings.  Another  example  is  given  in the discussion of DEFINE
       above.

       All subroutine calls, whether recursive or not, are always  treated  as
       atomic  groups. That is, once a subroutine has matched some of the sub-
       ject string, it is never re-entered, even if it contains untried alter-
       natives  and  there  is  a  subsequent  matching failure. Any capturing
       parentheses that are set during the subroutine  call  revert  to  their
       previous values afterwards.

       Processing  options  such as case-independence are fixed when a subpat-
       tern is defined, so if it is used as a subroutine, such options  cannot
       be changed for different calls. For example, consider this pattern:

         (abc)(?i:(?-1))

       It  matches  "abcabc". It does not match "abcABC" because the change of
       processing option does not affect the called subpattern.


ONIGURUMA SUBROUTINE SYNTAX

       For compatibility with Oniguruma, the non-Perl syntax \g followed by  a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for referencing a  subpattern  as  a  subroutine,
       possibly  recursively. Here are two of the examples used above, rewrit-
       ten using this syntax:

         (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
         (sens|respons)e and \g'1'ibility

       PCRE supports an extension to Oniguruma: if a number is preceded  by  a
       plus or a minus sign it is taken as a relative reference. For example:

         (abc)(?i:\g<-1>)

       Note  that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
       synonymous. The former is a back reference; the latter is a  subroutine
       call.


CALLOUTS

       Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl code to be obeyed in the middle of matching a regular  expression.
       This makes it possible, amongst other things, to extract different sub-
       strings that match the same pair of parentheses when there is a repeti-
       tion.

       PCRE provides a similar feature, but of course it cannot obey arbitrary
       Perl code. The feature is called "callout". The caller of PCRE provides
       an  external function by putting its entry point in the global variable
       pcre_callout (8-bit library) or  pcre16_callout  (16-bit  library).  By
       default, this variable contains NULL, which disables all calling out.

       Within  a  regular  expression,  (?C) indicates the points at which the
       external function is to be called. If you want  to  identify  different
       callout  points, you can put a number less than 256 after the letter C.
       The default value is zero.  For example, this pattern has  two  callout
       points:

         (?C1)abc(?C2)def

       If  the PCRE_AUTO_CALLOUT flag is passed to a compiling function, call-
       outs are automatically installed before each item in the pattern.  They
       are all numbered 255.

       During  matching, when PCRE reaches a callout point, the external func-
       tion is called. It is provided with the  number  of  the  callout,  the
       position  in  the pattern, and, optionally, one item of data originally
       supplied by the caller of the matching function. The  callout  function
       may  cause  matching to proceed, to backtrack, or to fail altogether. A
       complete description of the interface to the callout function is  given
       in the pcrecallout documentation.


BACKTRACKING CONTROL

       Perl  5.10 introduced a number of "Special Backtracking Control Verbs",
       which are described in the Perl documentation as "experimental and sub-
       ject  to  change or removal in a future version of Perl". It goes on to
       say: "Their usage in production code should be noted to avoid  problems
       during upgrades." The same remarks apply to the PCRE features described
       in this section.

       Since these verbs are specifically related  to  backtracking,  most  of
       them  can  be  used only when the pattern is to be matched using one of
       the traditional matching functions, which use a backtracking algorithm.
       With  the  exception  of (*FAIL), which behaves like a failing negative
       assertion, they cause an error if encountered by a DFA  matching  func-
       tion.

       If  any of these verbs are used in an assertion or in a subpattern that
       is called as a subroutine (whether or not recursively), their effect is
       confined to that subpattern; it does not extend to the surrounding pat-
       tern, with one exception: the name from a *(MARK), (*PRUNE), or (*THEN)
       that  is  encountered in a successful positive assertion is passed back
       when a match succeeds (compare capturing  parentheses  in  assertions).
       Note that such subpatterns are processed as anchored at the point where
       they are tested. Note also that Perl's  treatment  of  subroutines  and
       assertions is different in some cases.

       The  new verbs make use of what was previously invalid syntax: an open-
       ing parenthesis followed by an asterisk. They are generally of the form
       (*VERB)  or (*VERB:NAME). Some may take either form, with differing be-
       haviour, depending on whether or not an argument is present. A name  is
       any sequence of characters that does not include a closing parenthesis.
       The maximum length of name is 255 in the 8-bit library and 65535 in the
       16-bit library. If the name is empty, that is, if the closing parenthe-
       sis immediately follows the colon, the effect is as if the  colon  were
       not there. Any number of these verbs may occur in a pattern.

   Optimizations that affect backtracking verbs

       PCRE  contains some optimizations that are used to speed up matching by
       running some checks at the start of each match attempt. For example, it
       may  know  the minimum length of matching subject, or that a particular
       character must be present. When one of these  optimizations  suppresses
       the  running  of  a match, any included backtracking verbs will not, of
       course, be processed. You can suppress the start-of-match optimizations
       by  setting  the  PCRE_NO_START_OPTIMIZE  option when calling pcre_com-
       pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT).
       There is more discussion of this option in the section entitled "Option
       bits for pcre_exec()" in the pcreapi documentation.

       Experiments with Perl suggest that it too  has  similar  optimizations,
       sometimes leading to anomalous results.

   Verbs that act immediately

       The  following  verbs act as soon as they are encountered. They may not
       be followed by a name.

          (*ACCEPT)

       This verb causes the match to end successfully, skipping the  remainder
       of  the pattern. However, when it is inside a subpattern that is called
       as a subroutine, only that subpattern is ended  successfully.  Matching
       then  continues  at  the  outer level. If (*ACCEPT) is inside capturing
       parentheses, the data so far is captured. For example:

         A((?:A|B(*ACCEPT)|C)D)

       This matches "AB", "AAD", or "ACD"; when it matches "AB", "B"  is  cap-
       tured by the outer parentheses.

         (*FAIL) or (*F)

       This  verb causes a matching failure, forcing backtracking to occur. It
       is equivalent to (?!) but easier to read. The Perl documentation  notes
       that  it  is  probably  useful only when combined with (?{}) or (??{}).
       Those are, of course, Perl features that are not present in  PCRE.  The
       nearest  equivalent is the callout feature, as for example in this pat-
       tern:

         a+(?C)(*FAIL)

       A match with the string "aaaa" always fails, but the callout  is  taken
       before each backtrack happens (in this example, 10 times).

   Recording which path was taken

       There  is  one  verb  whose  main  purpose  is to track how a match was
       arrived at, though it also has a  secondary  use  in  conjunction  with
       advancing the match starting point (see (*SKIP) below).

         (*MARK:NAME) or (*:NAME)

       A  name  is  always  required  with  this  verb.  There  may be as many
       instances of (*MARK) as you like in a pattern, and their names  do  not
       have to be unique.

       When  a match succeeds, the name of the last-encountered (*MARK) on the
       matching path is passed back to the caller as described in the  section
       entitled  "Extra  data  for  pcre_exec()" in the pcreapi documentation.
       Here is an example of pcretest output, where the /K  modifier  requests
       the retrieval and outputting of (*MARK) data:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/K
         data> XY
          0: XY
         MK: A
         XZ
          0: XZ
         MK: B

       The (*MARK) name is tagged with "MK:" in this output, and in this exam-
       ple it indicates which of the two alternatives matched. This is a  more
       efficient  way of obtaining this information than putting each alterna-
       tive in its own capturing parentheses.

       If (*MARK) is encountered in a positive assertion, its name is recorded
       and passed back if it is the last-encountered. This does not happen for
       negative assertions.

       After a partial match or a failed match, the name of the  last  encoun-
       tered (*MARK) in the entire match process is returned. For example:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/K
         data> XP
         No match, mark = B

       Note  that  in  this  unanchored  example the mark is retained from the
       match attempt that started at the letter "X" in the subject. Subsequent
       match attempts starting at "P" and then with an empty string do not get
       as far as the (*MARK) item, but nevertheless do not reset it.

       If you are interested in  (*MARK)  values  after  failed  matches,  you
       should  probably  set  the PCRE_NO_START_OPTIMIZE option (see above) to
       ensure that the match is always attempted.

   Verbs that act after backtracking

       The following verbs do nothing when they are encountered. Matching con-
       tinues  with what follows, but if there is no subsequent match, causing
       a backtrack to the verb, a failure is  forced.  That  is,  backtracking
       cannot  pass  to the left of the verb. However, when one of these verbs
       appears inside an atomic group, its effect is confined to  that  group,
       because  once the group has been matched, there is never any backtrack-
       ing into it. In this situation, backtracking can  "jump  back"  to  the
       left  of the entire atomic group. (Remember also, as stated above, that
       this localization also applies in subroutine calls and assertions.)

       These verbs differ in exactly what kind of failure  occurs  when  back-
       tracking reaches them.

         (*COMMIT)

       This  verb, which may not be followed by a name, causes the whole match
       to fail outright if the rest of the pattern does not match. Even if the
       pattern is unanchored, no further attempts to find a match by advancing
       the  starting  point  take  place.  Once  (*COMMIT)  has  been  passed,
       pcre_exec()  is  committed  to  finding a match at the current starting
       point, or not at all. For example:

         a+(*COMMIT)b

       This matches "xxaab" but not "aacaab". It can be thought of as  a  kind
       of dynamic anchor, or "I've started, so I must finish." The name of the
       most recently passed (*MARK) in the path is passed back when  (*COMMIT)
       forces a match failure.

       Note  that  (*COMMIT)  at  the start of a pattern is not the same as an
       anchor, unless PCRE's start-of-match optimizations are turned  off,  as
       shown in this pcretest example:

           re> /(*COMMIT)abc/
         data> xyzabc
          0: abc
         xyzabc\Y
         No match

       PCRE  knows  that  any  match  must start with "a", so the optimization
       skips along the subject to "a" before running the first match  attempt,
       which  succeeds.  When the optimization is disabled by the \Y escape in
       the second subject, the match starts at "x" and so the (*COMMIT) causes
       it to fail without trying any other starting points.

         (*PRUNE) or (*PRUNE:NAME)

       This  verb causes the match to fail at the current starting position in
       the subject if the rest of the pattern does not match. If  the  pattern
       is  unanchored,  the  normal  "bumpalong"  advance to the next starting
       character then happens. Backtracking can occur as usual to the left  of
       (*PRUNE),  before  it  is  reached,  or  when  matching to the right of
       (*PRUNE), but if there is no match to the  right,  backtracking  cannot
       cross  (*PRUNE). In simple cases, the use of (*PRUNE) is just an alter-
       native to an atomic group or possessive quantifier, but there are  some
       uses of (*PRUNE) that cannot be expressed in any other way.  The behav-
       iour of (*PRUNE:NAME)  is  the  same  as  (*MARK:NAME)(*PRUNE).  In  an
       anchored pattern (*PRUNE) has the same effect as (*COMMIT).

         (*SKIP)

       This  verb, when given without a name, is like (*PRUNE), except that if
       the pattern is unanchored, the "bumpalong" advance is not to  the  next
       character, but to the position in the subject where (*SKIP) was encoun-
       tered. (*SKIP) signifies that whatever text was matched leading  up  to
       it cannot be part of a successful match. Consider:

         a+(*SKIP)b

       If  the  subject  is  "aaaac...",  after  the first match attempt fails
       (starting at the first character in the  string),  the  starting  point
       skips on to start the next attempt at "c". Note that a possessive quan-
       tifer does not have the same effect as this example; although it  would
       suppress  backtracking  during  the  first  match  attempt,  the second
       attempt would start at the second character instead of skipping  on  to
       "c".

         (*SKIP:NAME)

       When  (*SKIP) has an associated name, its behaviour is modified. If the
       following pattern fails to match, the previous path through the pattern
       is  searched for the most recent (*MARK) that has the same name. If one
       is found, the "bumpalong" advance is to the subject position that  cor-
       responds  to  that (*MARK) instead of to where (*SKIP) was encountered.
       If no (*MARK) with a matching name is found, the (*SKIP) is ignored.

         (*THEN) or (*THEN:NAME)

       This verb causes a skip to the next innermost alternative if  the  rest
       of  the  pattern does not match. That is, it cancels pending backtrack-
       ing, but only within the current alternative. Its name comes  from  the
       observation that it can be used for a pattern-based if-then-else block:

         ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       If  the COND1 pattern matches, FOO is tried (and possibly further items
       after the end of the group if FOO succeeds); on  failure,  the  matcher
       skips  to  the second alternative and tries COND2, without backtracking
       into COND1. The behaviour  of  (*THEN:NAME)  is  exactly  the  same  as
       (*MARK:NAME)(*THEN).   If (*THEN) is not inside an alternation, it acts
       like (*PRUNE).

       Note that a subpattern that does not contain a | character  is  just  a
       part  of the enclosing alternative; it is not a nested alternation with
       only one alternative. The effect of (*THEN) extends beyond such a  sub-
       pattern  to  the enclosing alternative. Consider this pattern, where A,
       B, etc. are complex pattern fragments that do not contain any | charac-
       ters at this level:

         A (B(*THEN)C) | D

       If  A and B are matched, but there is a failure in C, matching does not
       backtrack into A; instead it moves to the next alternative, that is, D.
       However,  if the subpattern containing (*THEN) is given an alternative,
       it behaves differently:

         A (B(*THEN)C | (*FAIL)) | D

       The effect of (*THEN) is now confined to the inner subpattern. After  a
       failure in C, matching moves to (*FAIL), which causes the whole subpat-
       tern to fail because there are no more alternatives  to  try.  In  this
       case, matching does now backtrack into A.

       Note also that a conditional subpattern is not considered as having two
       alternatives, because only one is ever used.  In  other  words,  the  |
       character in a conditional subpattern has a different meaning. Ignoring
       white space, consider:

         ^.*? (?(?=a) a | b(*THEN)c )

       If the subject is "ba", this pattern does not  match.  Because  .*?  is
       ungreedy,  it  initially  matches  zero characters. The condition (?=a)
       then fails, the character "b" is matched,  but  "c"  is  not.  At  this
       point,  matching does not backtrack to .*? as might perhaps be expected
       from the presence of the | character.  The  conditional  subpattern  is
       part of the single alternative that comprises the whole pattern, and so
       the match fails. (If there was a backtrack into  .*?,  allowing  it  to
       match "b", the match would succeed.)

       The  verbs just described provide four different "strengths" of control
       when subsequent matching fails. (*THEN) is the weakest, carrying on the
       match  at  the next alternative. (*PRUNE) comes next, failing the match
       at the current starting position, but allowing an advance to  the  next
       character  (for an unanchored pattern). (*SKIP) is similar, except that
       the advance may be more than one character. (*COMMIT) is the strongest,
       causing the entire match to fail.

       If more than one such verb is present in a pattern, the "strongest" one
       wins.  For example, consider this pattern, where A, B, etc. are complex
       pattern fragments:

         (A(*COMMIT)B(*THEN)C|D)

       Once  A  has  matched,  PCRE is committed to this match, at the current
       starting position. If subsequently B matches, but C does not, the  nor-
       mal (*THEN) action of trying the next alternative (that is, D) does not
       happen because (*COMMIT) overrides.


SEE ALSO

       pcreapi(3), pcrecallout(3),  pcrematching(3),  pcresyntax(3),  pcre(3),
       pcre16(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 01 June 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRESYNTAX(3)                                                    PCRESYNTAX(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE REGULAR EXPRESSION SYNTAX SUMMARY

       The  full syntax and semantics of the regular expressions that are sup-
       ported by PCRE are described in  the  pcrepattern  documentation.  This
       document contains a quick-reference summary of the syntax.


QUOTING

         \x         where x is non-alphanumeric is a literal x
         \Q...\E    treat enclosed characters as literal


CHARACTERS

         \a         alarm, that is, the BEL character (hex 07)
         \cx        "control-x", where x is any ASCII character
         \e         escape (hex 1B)
         \f         form feed (hex 0C)
         \n         newline (hex 0A)
         \r         carriage return (hex 0D)
         \t         tab (hex 09)
         \ddd       character with octal code ddd, or backreference
         \xhh       character with hex code hh
         \x{hhh..}  character with hex code hhh..


CHARACTER TYPES

         .          any character except newline;
                      in dotall mode, any character whatsoever
         \C         one data unit, even in UTF mode (best avoided)
         \d         a decimal digit
         \D         a character that is not a decimal digit
         \h         a horizontal white space character
         \H         a character that is not a horizontal white space character
         \N         a character that is not a newline
         \p{xx}     a character with the xx property
         \P{xx}     a character without the xx property
         \R         a newline sequence
         \s         a white space character
         \S         a character that is not a white space character
         \v         a vertical white space character
         \V         a character that is not a vertical white space character
         \w         a "word" character
         \W         a "non-word" character
         \X         an extended Unicode sequence

       In  PCRE,  by  default, \d, \D, \s, \S, \w, and \W recognize only ASCII
       characters, even in a UTF mode. However, this can be changed by setting
       the PCRE_UCP option.


GENERAL CATEGORY PROPERTIES FOR \p and \P

         C          Other
         Cc         Control
         Cf         Format
         Cn         Unassigned
         Co         Private use
         Cs         Surrogate

         L          Letter
         Ll         Lower case letter
         Lm         Modifier letter
         Lo         Other letter
         Lt         Title case letter
         Lu         Upper case letter
         L&         Ll, Lu, or Lt

         M          Mark
         Mc         Spacing mark
         Me         Enclosing mark
         Mn         Non-spacing mark

         N          Number
         Nd         Decimal number
         Nl         Letter number
         No         Other number

         P          Punctuation
         Pc         Connector punctuation
         Pd         Dash punctuation
         Pe         Close punctuation
         Pf         Final punctuation
         Pi         Initial punctuation
         Po         Other punctuation
         Ps         Open punctuation

         S          Symbol
         Sc         Currency symbol
         Sk         Modifier symbol
         Sm         Mathematical symbol
         So         Other symbol

         Z          Separator
         Zl         Line separator
         Zp         Paragraph separator
         Zs         Space separator


PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P

         Xan        Alphanumeric: union of properties L and N
         Xps        POSIX space: property Z or tab, NL, VT, FF, CR
         Xsp        Perl space: property Z or tab, NL, FF, CR
         Xwd        Perl word: property Xan or underscore


SCRIPT NAMES FOR \p AND \P

       Arabic,  Armenian,  Avestan, Balinese, Bamum, Batak, Bengali, Bopomofo,
       Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian,  Chakma,
       Cham,  Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret,
       Devanagari,  Egyptian_Hieroglyphs,  Ethiopic,   Georgian,   Glagolitic,
       Gothic,  Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
       gana,  Imperial_Aramaic,  Inherited,  Inscriptional_Pahlavi,   Inscrip-
       tional_Parthian,   Javanese,   Kaithi,   Kannada,  Katakana,  Kayah_Li,
       Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B,  Lisu,  Lycian,
       Lydian,    Malayalam,    Mandaic,    Meetei_Mayek,    Meroitic_Cursive,
       Meroitic_Hieroglyphs,  Miao,  Mongolian,  Myanmar,  New_Tai_Lue,   Nko,
       Ogham,    Old_Italic,   Old_Persian,   Old_South_Arabian,   Old_Turkic,
       Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic,  Samari-
       tan,  Saurashtra,  Sharada,  Shavian, Sinhala, Sora_Sompeng, Sundanese,
       Syloti_Nagri, Syriac, Tagalog, Tagbanwa,  Tai_Le,  Tai_Tham,  Tai_Viet,
       Takri,  Tamil,  Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai,
       Yi.


CHARACTER CLASSES

         [...]       positive character class
         [^...]      negative character class
         [x-y]       range (can be used for hex characters)
         [[:xxx:]]   positive POSIX named set
         [[:^xxx:]]  negative POSIX named set

         alnum       alphanumeric
         alpha       alphabetic
         ascii       0-127
         blank       space or tab
         cntrl       control character
         digit       decimal digit
         graph       printing, excluding space
         lower       lower case letter
         print       printing, including space
         punct       printing, excluding alphanumeric
         space       white space
         upper       upper case letter
         word        same as \w
         xdigit      hexadecimal digit

       In PCRE, POSIX character set names recognize only ASCII  characters  by
       default,  but  some  of them use Unicode properties if PCRE_UCP is set.
       You can use \Q...\E inside a character class.


QUANTIFIERS

         ?           0 or 1, greedy
         ?+          0 or 1, possessive
         ??          0 or 1, lazy
         *           0 or more, greedy
         *+          0 or more, possessive
         *?          0 or more, lazy
         +           1 or more, greedy
         ++          1 or more, possessive
         +?          1 or more, lazy
         {n}         exactly n
         {n,m}       at least n, no more than m, greedy
         {n,m}+      at least n, no more than m, possessive
         {n,m}?      at least n, no more than m, lazy
         {n,}        n or more, greedy
         {n,}+       n or more, possessive
         {n,}?       n or more, lazy


ANCHORS AND SIMPLE ASSERTIONS

         \b          word boundary
         \B          not a word boundary
         ^           start of subject
                      also after internal newline in multiline mode
         \A          start of subject
         $           end of subject
                      also before newline at end of subject
                      also before internal newline in multiline mode
         \Z          end of subject
                      also before newline at end of subject
         \z          end of subject
         \G          first matching position in subject


MATCH POINT RESET

         \K          reset start of match


ALTERNATION

         expr|expr|expr...


CAPTURING

         (...)           capturing group
         (?<name>...)    named capturing group (Perl)
         (?'name'...)    named capturing group (Perl)
         (?P<name>...)   named capturing group (Python)
         (?:...)         non-capturing group
         (?|...)         non-capturing group; reset group numbers for
                          capturing groups in each alternative


ATOMIC GROUPS

         (?>...)         atomic, non-capturing group


COMMENT

         (?#....)        comment (not nestable)


OPTION SETTING

         (?i)            caseless
         (?J)            allow duplicate names
         (?m)            multiline
         (?s)            single line (dotall)
         (?U)            default ungreedy (lazy)
         (?x)            extended (ignore white space)
         (?-...)         unset option(s)

       The following are recognized only at the start of a  pattern  or  after
       one of the newline-setting options with similar syntax:

         (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE)
         (*UTF8)         set UTF-8 mode: 8-bit library (PCRE_UTF8)
         (*UTF16)        set UTF-16 mode: 16-bit library (PCRE_UTF16)
         (*UCP)          set PCRE_UCP (use Unicode properties for \d etc)


LOOKAHEAD AND LOOKBEHIND ASSERTIONS

         (?=...)         positive look ahead
         (?!...)         negative look ahead
         (?<=...)        positive look behind
         (?<!...)        negative look behind

       Each top-level branch of a look behind must be of a fixed length.


BACKREFERENCES

         \n              reference by number (can be ambiguous)
         \gn             reference by number
         \g{n}           reference by number
         \g{-n}          relative reference by number
         \k<name>        reference by name (Perl)
         \k'name'        reference by name (Perl)
         \g{name}        reference by name (Perl)
         \k{name}        reference by name (.NET)
         (?P=name)       reference by name (Python)


SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)

         (?R)            recurse whole pattern
         (?n)            call subpattern by absolute number
         (?+n)           call subpattern by relative number
         (?-n)           call subpattern by relative number
         (?&name)        call subpattern by name (Perl)
         (?P>name)       call subpattern by name (Python)
         \g<name>        call subpattern by name (Oniguruma)
         \g'name'        call subpattern by name (Oniguruma)
         \g<n>           call subpattern by absolute number (Oniguruma)
         \g'n'           call subpattern by absolute number (Oniguruma)
         \g<+n>          call subpattern by relative number (PCRE extension)
         \g'+n'          call subpattern by relative number (PCRE extension)
         \g<-n>          call subpattern by relative number (PCRE extension)
         \g'-n'          call subpattern by relative number (PCRE extension)


CONDITIONAL PATTERNS

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

         (?(n)...        absolute reference condition
         (?(+n)...       relative reference condition
         (?(-n)...       relative reference condition
         (?(<name>)...   named reference condition (Perl)
         (?('name')...   named reference condition (Perl)
         (?(name)...     named reference condition (PCRE)
         (?(R)...        overall recursion condition
         (?(Rn)...       specific group recursion condition
         (?(R&name)...   specific recursion condition
         (?(DEFINE)...   define subpattern for reference
         (?(assert)...   assertion condition


BACKTRACKING CONTROL

       The following act immediately they are reached:

         (*ACCEPT)       force successful match
         (*FAIL)         force backtrack; synonym (*F)
         (*MARK:NAME)    set name to be passed back; synonym (*:NAME)

       The  following  act only when a subsequent match failure causes a back-
       track to reach them. They all force a match failure, but they differ in
       what happens afterwards. Those that advance the start-of-match point do
       so only if the pattern is not anchored.

         (*COMMIT)       overall failure, no advance of starting point
         (*PRUNE)        advance to next starting character
         (*PRUNE:NAME)   equivalent to (*MARK:NAME)(*PRUNE)
         (*SKIP)         advance to current matching position
         (*SKIP:NAME)    advance to position corresponding to an earlier
                         (*MARK:NAME); if not found, the (*SKIP) is ignored
         (*THEN)         local failure, backtrack to next alternation
         (*THEN:NAME)    equivalent to (*MARK:NAME)(*THEN)


NEWLINE CONVENTIONS

       These are recognized only at the very start of the pattern or  after  a
       (*BSR_...), (*UTF8), (*UTF16) or (*UCP) option.

         (*CR)           carriage return only
         (*LF)           linefeed only
         (*CRLF)         carriage return followed by linefeed
         (*ANYCRLF)      all three of the above
         (*ANY)          any Unicode newline sequence


WHAT \R MATCHES

       These  are  recognized only at the very start of the pattern or after a
       (*...) option that sets the newline convention or a UTF or UCP mode.

         (*BSR_ANYCRLF)  CR, LF, or CRLF
         (*BSR_UNICODE)  any Unicode newline sequence


CALLOUTS

         (?C)      callout
         (?Cn)     callout with data n


SEE ALSO

       pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 10 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREUNICODE(3)                                                  PCREUNICODE(3)


NAME
       PCRE - Perl-compatible regular expressions


UTF-8, UTF-16, AND UNICODE PROPERTY SUPPORT

       From Release 8.30, in addition to its previous UTF-8 support, PCRE also
       supports UTF-16 by means of a separate  16-bit  library.  This  can  be
       built as well as, or instead of, the 8-bit library.


UTF-8 SUPPORT

       In  order  process  UTF-8  strings, you must build PCRE's 8-bit library
       with UTF support, and, in addition, you must call  pcre_compile()  with
       the  PCRE_UTF8 option flag, or the pattern must start with the sequence
       (*UTF8). When either of these is the case, both  the  pattern  and  any
       subject  strings  that  are  matched  against  it  are treated as UTF-8
       strings instead of strings of 1-byte characters.


UTF-16 SUPPORT

       In order process UTF-16 strings, you must build PCRE's  16-bit  library
       with UTF support, and, in addition, you must call pcre16_compile() with
       the PCRE_UTF16 option flag, or the pattern must start with the sequence
       (*UTF16).  When  either  of these is the case, both the pattern and any
       subject strings that are matched  against  it  are  treated  as  UTF-16
       strings instead of strings of 16-bit characters.


UTF SUPPORT OVERHEAD

       If  you  compile  PCRE with UTF support, but do not use it at run time,
       the library will be a bit bigger, but the additional run time  overhead
       is limited to testing the PCRE_UTF8/16 flag occasionally, so should not
       be very big.


UNICODE PROPERTY SUPPORT

       If PCRE is built with Unicode character property support (which implies
       UTF  support), the escape sequences \p{..}, \P{..}, and \X can be used.
       The available properties that can be tested are limited to the  general
       category  properties  such  as  Lu for an upper case letter or Nd for a
       decimal number, the Unicode script names such as Arabic or Han, and the
       derived  properties Any and L&. A full list is given in the pcrepattern
       documentation. Only the short names for properties are  supported.  For
       example,  \p{L}  matches a letter. Its Perl synonym, \p{Letter}, is not
       supported.  Furthermore, in Perl, many  properties  may  optionally  be
       prefixed  by  "Is", for compatibility with Perl 5.6. PCRE does not sup-
       port this.

   Validity of UTF-8 strings

       When you set the PCRE_UTF8 flag, the byte strings  passed  as  patterns
       and subjects are (by default) checked for validity on entry to the rel-
       evant functions. The entire string is checked before any other process-
       ing  takes  place. From release 7.3 of PCRE, the check is according the
       rules of RFC 3629, which are themselves derived from the Unicode speci-
       fication.  Earlier  releases  of  PCRE  followed the rules of RFC 2279,
       which allows the full range of 31-bit values  (0  to  0x7FFFFFFF).  The
       current  check allows only values in the range U+0 to U+10FFFF, exclud-
       ing U+D800 to U+DFFF.

       The excluded code points are the "Surrogate Area" of Unicode. They  are
       reserved  for  use  by  UTF-16,  where they are used in pairs to encode
       codepoints with values greater than 0xFFFF. The code  points  that  are
       encoded by UTF-16 pairs are available independently in the UTF-8 encod-
       ing. (In other words, the whole surrogate thing is a fudge  for  UTF-16
       which unfortunately messes up UTF-8.)

       If an invalid UTF-8 string is passed to PCRE, an error return is given.
       At compile time, the only additional information is the offset  to  the
       first byte of the failing character. The run-time functions pcre_exec()
       and pcre_dfa_exec() also pass back this information, as well as a  more
       detailed  reason  code if the caller has provided memory in which to do
       this.

       In some situations, you may already know that your strings  are  valid,
       and  therefore  want  to  skip these checks in order to improve perfor-
       mance, for example in the case of a long subject string that  is  being
       scanned   repeatedly   with   different   patterns.   If  you  set  the
       PCRE_NO_UTF8_CHECK flag at compile time or at run  time,  PCRE  assumes
       that  the  pattern  or subject it is given (respectively) contains only
       valid UTF-8 codes. In this case, it does not diagnose an invalid  UTF-8
       string.

       If  you  pass  an  invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
       what happens depends on why the string is invalid. If the  string  con-
       forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
       string of characters in the range 0 to  0x7FFFFFFF  by  pcre_dfa_exec()
       and  the interpreted version of pcre_exec(). In other words, apart from
       the initial validity test, these functions (when in UTF-8 mode)  handle
       strings  according  to the more liberal rules of RFC 2279. However, the
       just-in-time (JIT) optimization for pcre_exec() supports only RFC 3629.
       If  you are using JIT optimization, or if the string does not even con-
       form to RFC 2279, the result is undefined. Your program may crash.

       If you want to process strings  of  values  in  the  full  range  0  to
       0x7FFFFFFF,  encoded in a UTF-8-like manner as per the old RFC, you can
       set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
       this  situation,  you  will  have to apply your own validity check, and
       avoid the use of JIT optimization.

   Validity of UTF-16 strings

       When you set the PCRE_UTF16 flag, the strings of 16-bit data units that
       are passed as patterns and subjects are (by default) checked for valid-
       ity on entry to the relevant functions. Values other than those in  the
       surrogate range U+D800 to U+DFFF are independent code points. Values in
       the surrogate range must be used in pairs in the correct manner.

       If an invalid UTF-16 string is passed  to  PCRE,  an  error  return  is
       given.  At  compile time, the only additional information is the offset
       to the first data unit of the failing character. The run-time functions
       pcre16_exec() and pcre16_dfa_exec() also pass back this information, as
       well as a more detailed reason code if the caller has  provided  memory
       in which to do this.

       In  some  situations, you may already know that your strings are valid,
       and therefore want to skip these checks in  order  to  improve  perfor-
       mance.  If  you  set the PCRE_NO_UTF16_CHECK flag at compile time or at
       run time, PCRE assumes that the pattern or subject it is given (respec-
       tively) contains only valid UTF-16 sequences. In this case, it does not
       diagnose an invalid UTF-16 string.

   General comments about UTF modes

       1. Codepoints less than 256  can  be  specified  by  either  braced  or
       unbraced  hexadecimal  escape  sequences (for example, \x{b3} or \xb3).
       Larger values have to use braced sequences.

       2. Octal numbers up to \777 are recognized, and  in  UTF-8  mode,  they
       match two-byte characters for values greater than \177.

       3. Repeat quantifiers apply to complete UTF characters, not to individ-
       ual data units, for example: \x{100}{3}.

       4. The dot metacharacter matches one UTF character instead of a  single
       data unit.

       5.  The  escape sequence \C can be used to match a single byte in UTF-8
       mode, or a single 16-bit data unit in UTF-16 mode, but its use can lead
       to some strange effects because it breaks up multi-unit characters (see
       the description of \C in the pcrepattern documentation). The use of  \C
       is    not    supported    in    the   alternative   matching   function
       pcre[16]_dfa_exec(), nor is it supported in UTF mode by the  JIT  opti-
       mization of pcre[16]_exec(). If JIT optimization is requested for a UTF
       pattern that contains \C, it will not succeed, and so the matching will
       be carried out by the normal interpretive function.

       6.  The  character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
       test characters of any code value, but, by default, the characters that
       PCRE  recognizes  as digits, spaces, or word characters remain the same
       set as in non-UTF mode, all with values less  than  256.  This  remains
       true  even  when  PCRE  is  built  to include Unicode property support,
       because to do otherwise would slow down PCRE in many common cases. Note
       in  particular that this applies to \b and \B, because they are defined
       in terms of \w and \W. If you really want to test for a wider sense of,
       say,  "digit",  you  can  use  explicit  Unicode property tests such as
       \p{Nd}. Alternatively, if you set the PCRE_UCP option, the way that the
       character  escapes  work is changed so that Unicode properties are used
       to determine which characters match. There are more details in the sec-
       tion on generic character types in the pcrepattern documentation.

       7.  Similarly,  characters that match the POSIX named character classes
       are all low-valued characters, unless the PCRE_UCP option is set.

       8. However, the horizontal and vertical white  space  matching  escapes
       (\h,  \H,  \v, and \V) do match all the appropriate Unicode characters,
       whether or not PCRE_UCP is set.

       9. Case-insensitive matching applies only to  characters  whose  values
       are  less than 128, unless PCRE is built with Unicode property support.
       Even when Unicode property support is available, PCRE  still  uses  its
       own  character  tables when checking the case of low-valued characters,
       so as not to degrade performance.  The Unicode property information  is
       used only for characters with higher values. Furthermore, PCRE supports
       case-insensitive matching only  when  there  is  a  one-to-one  mapping
       between  a letter's cases. There are a small number of many-to-one map-
       pings in Unicode; these are not supported by PCRE.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 14 April 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREJIT(3)                                                          PCREJIT(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE JUST-IN-TIME COMPILER SUPPORT

       Just-in-time  compiling  is a heavyweight optimization that can greatly
       speed up pattern matching. However, it comes at the cost of extra  pro-
       cessing before the match is performed. Therefore, it is of most benefit
       when the same pattern is going to be matched many times. This does  not
       necessarily  mean  many calls of a matching function; if the pattern is
       not anchored, matching attempts may take place many  times  at  various
       positions  in  the  subject, even for a single call.  Therefore, if the
       subject string is very long, it may still pay to use  JIT  for  one-off
       matches.

       JIT  support  applies  only to the traditional Perl-compatible matching
       function.  It does not apply when the DFA matching  function  is  being
       used. The code for this support was written by Zoltan Herczeg.


8-BIT and 16-BIT SUPPORT

       JIT  support is available for both the 8-bit and 16-bit PCRE libraries.
       To  keep  this  documentation  simple,  only  the  8-bit  interface  is
       described in what follows. If you are using the 16-bit library, substi-
       tute  the  16-bit  functions  and  16-bit  structures   (for   example,
       pcre16_jit_stack instead of pcre_jit_stack).


AVAILABILITY OF JIT SUPPORT

       JIT  support  is  an  optional  feature of PCRE. The "configure" option
       --enable-jit (or equivalent CMake option) must  be  set  when  PCRE  is
       built  if  you want to use JIT. The support is limited to the following
       hardware platforms:

         ARM v5, v7, and Thumb2
         Intel x86 32-bit and 64-bit
         MIPS 32-bit
         Power PC 32-bit and 64-bit

       If --enable-jit is set on an unsupported platform, compilation fails.

       A program that is linked with PCRE 8.20 or later can tell if  JIT  sup-
       port  is  available  by  calling pcre_config() with the PCRE_CONFIG_JIT
       option. The result is 1 when JIT is available, and  0  otherwise.  How-
       ever, a simple program does not need to check this in order to use JIT.
       The API is implemented in a way that falls  back  to  the  interpretive
       code if JIT is not available.

       If  your program may sometimes be linked with versions of PCRE that are
       older than 8.20, but you want to use JIT when it is available, you  can
       test the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT
       macro such as PCRE_CONFIG_JIT, for compile-time control of your code.


SIMPLE USE OF JIT

       You have to do two things to make use of the JIT support  in  the  sim-
       plest way:

         (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for
             each compiled pattern, and pass the resulting pcre_extra block to
             pcre_exec().

         (2) Use pcre_free_study() to free the pcre_extra block when it is
             no longer needed, instead of just freeing it yourself. This
             ensures that any JIT data is also freed.

       For  a  program  that may be linked with pre-8.20 versions of PCRE, you
       can insert

         #ifndef PCRE_STUDY_JIT_COMPILE
         #define PCRE_STUDY_JIT_COMPILE 0
         #endif

       so that no option is passed to pcre_study(),  and  then  use  something
       like this to free the study data:

         #ifdef PCRE_CONFIG_JIT
             pcre_free_study(study_ptr);
         #else
             pcre_free(study_ptr);
         #endif

       PCRE_STUDY_JIT_COMPILE  requests  the JIT compiler to generate code for
       complete matches.  If  you  want  to  run  partial  matches  using  the
       PCRE_PARTIAL_HARD  or  PCRE_PARTIAL_SOFT  options  of  pcre_exec(), you
       should set one or both of the following  options  in  addition  to,  or
       instead of, PCRE_STUDY_JIT_COMPILE when you call pcre_study():

         PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
         PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE

       The  JIT  compiler  generates  different optimized code for each of the
       three modes (normal, soft partial, hard partial). When  pcre_exec()  is
       called,  the appropriate code is run if it is available. Otherwise, the
       pattern is matched using interpretive code.

       In some circumstances you may need to call additional functions.  These
       are  described  in  the  section  entitled  "Controlling the JIT stack"
       below.

       If JIT  support  is  not  available,  PCRE_STUDY_JIT_COMPILE  etc.  are
       ignored, and no JIT data is created. Otherwise, the compiled pattern is
       passed to the JIT compiler, which turns it into machine code that  exe-
       cutes  much  faster than the normal interpretive code. When pcre_exec()
       is passed a pcre_extra block containing a pointer to JIT  code  of  the
       appropriate  mode  (normal  or  hard/soft  partial), it obeys that code
       instead of running the interpreter. The result is  identical,  but  the
       compiled JIT code runs much faster.

       There  are some pcre_exec() options that are not supported for JIT exe-
       cution. There are also some  pattern  items  that  JIT  cannot  handle.
       Details  are  given below. In both cases, execution automatically falls
       back to the interpretive code. If you want  to  know  whether  JIT  was
       actually  used  for  a  particular  match, you should arrange for a JIT
       callback function to be set up as described  in  the  section  entitled
       "Controlling  the JIT stack" below, even if you do not need to supply a
       non-default JIT stack. Such a callback function is called whenever  JIT
       code  is about to be obeyed. If the execution options are not right for
       JIT execution, the callback function is not obeyed.

       If the JIT compiler finds an unsupported item, no JIT  data  is  gener-
       ated.  You  can find out if JIT execution is available after studying a
       pattern by calling pcre_fullinfo() with  the  PCRE_INFO_JIT  option.  A
       result  of  1  means that JIT compilation was successful. A result of 0
       means that JIT support is not available, or the pattern was not studied
       with  PCRE_STUDY_JIT_COMPILE  etc., or the JIT compiler was not able to
       handle the pattern.

       Once a pattern has been studied, with or without JIT, it can be used as
       many times as you like for matching different subject strings.


UNSUPPORTED OPTIONS AND PATTERN ITEMS

       The  only  pcre_exec() options that are supported for JIT execution are
       PCRE_NO_UTF8_CHECK,  PCRE_NO_UTF16_CHECK,   PCRE_NOTBOL,   PCRE_NOTEOL,
       PCRE_NOTEMPTY,  PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PAR-
       TIAL_SOFT.

       The unsupported pattern items are:

         \C             match a single byte; not supported in UTF-8 mode
         (?Cn)          callouts
         (*PRUNE)       )
         (*SKIP)        ) backtracking control verbs
         (*THEN)        )

       Support for some of these may be added in future.


RETURN VALUES FROM JIT EXECUTION

       When a pattern is matched using JIT execution, the  return  values  are
       the  same as those given by the interpretive pcre_exec() code, with the
       addition of one new error code: PCRE_ERROR_JIT_STACKLIMIT.  This  means
       that  the memory used for the JIT stack was insufficient. See "Control-
       ling the JIT stack" below for a discussion of JIT stack usage. For com-
       patibility  with  the  interpretive pcre_exec() code, no more than two-
       thirds of the ovector argument is used for passing back  captured  sub-
       strings.

       The  error  code  PCRE_ERROR_MATCHLIMIT  is returned by the JIT code if
       searching a very large pattern tree goes on for too long, as it  is  in
       the  same circumstance when JIT is not used, but the details of exactly
       what is counted are not the same. The  PCRE_ERROR_RECURSIONLIMIT  error
       code is never returned by JIT execution.


SAVING AND RESTORING COMPILED PATTERNS

       The  code  that  is  generated by the JIT compiler is architecture-spe-
       cific, and is also position dependent. For those reasons it  cannot  be
       saved  (in a file or database) and restored later like the bytecode and
       other data of a compiled pattern. Saving and  restoring  compiled  pat-
       terns  is not something many people do. More detail about this facility
       is given in the pcreprecompile documentation. It should be possible  to
       run  pcre_study() on a saved and restored pattern, and thereby recreate
       the JIT data, but because JIT compilation uses  significant  resources,
       it  is  probably  not worth doing this; you might as well recompile the
       original pattern.


CONTROLLING THE JIT STACK

       When the compiled JIT code runs, it needs a block of memory to use as a
       stack.   By  default,  it  uses 32K on the machine stack. However, some
       large  or  complicated  patterns  need  more  than  this.   The   error
       PCRE_ERROR_JIT_STACKLIMIT  is  given  when  there  is not enough stack.
       Three functions are provided for managing blocks of memory for  use  as
       JIT  stacks. There is further discussion about the use of JIT stacks in
       the section entitled "JIT stack FAQ" below.

       The pcre_jit_stack_alloc() function creates a JIT stack. Its  arguments
       are  a starting size and a maximum size, and it returns a pointer to an
       opaque structure of type pcre_jit_stack, or NULL if there is an  error.
       The  pcre_jit_stack_free() function can be used to free a stack that is
       no longer needed. (For the technically minded:  the  address  space  is
       allocated by mmap or VirtualAlloc.)

       JIT  uses far less memory for recursion than the interpretive code, and
       a maximum stack size of 512K to 1M should be more than enough  for  any
       pattern.

       The  pcre_assign_jit_stack()  function  specifies  which stack JIT code
       should use. Its arguments are as follows:

         pcre_extra         *extra
         pcre_jit_callback  callback
         void               *data

       The extra argument must be  the  result  of  studying  a  pattern  with
       PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the
       other two options:

         (1) If callback is NULL and data is NULL, an internal 32K block
             on the machine stack is used.

         (2) If callback is NULL and data is not NULL, data must be
             a valid JIT stack, the result of calling pcre_jit_stack_alloc().

         (3) If callback is not NULL, it must point to a function that is
             called with data as an argument at the start of matching, in
             order to set up a JIT stack. If the return from the callback
             function is NULL, the internal 32K stack is used; otherwise the
             return value must be a valid JIT stack, the result of calling
             pcre_jit_stack_alloc().

       A callback function is obeyed whenever JIT code is about to be run;  it
       is  not  obeyed when pcre_exec() is called with options that are incom-
       patible for JIT execution. A callback function can therefore be used to
       determine  whether  a  match  operation  was  executed by JIT or by the
       interpreter.

       You may safely use the same JIT stack for more than one pattern (either
       by  assigning directly or by callback), as long as the patterns are all
       matched sequentially in the same thread. In a multithread  application,
       if  you  do not specify a JIT stack, or if you assign or pass back NULL
       from a callback, that is thread-safe, because each thread has  its  own
       machine  stack.  However,  if  you  assign  or pass back a non-NULL JIT
       stack, this must be a different stack  for  each  thread  so  that  the
       application is thread-safe.

       Strictly  speaking,  even more is allowed. You can assign the same non-
       NULL stack to any number of patterns as long as they are not  used  for
       matching  by  multiple  threads  at the same time. For example, you can
       assign the same stack to all compiled patterns, and use a global  mutex
       in  the callback to wait until the stack is available for use. However,
       this is an inefficient solution, and not recommended.

       This is a suggestion for how a multithreaded program that needs to  set
       up non-default JIT stacks might operate:

         During thread initalization
           thread_local_var = pcre_jit_stack_alloc(...)

         During thread exit
           pcre_jit_stack_free(thread_local_var)

         Use a one-line callback function
           return thread_local_var

       All  the  functions  described in this section do nothing if JIT is not
       available, and pcre_assign_jit_stack() does nothing  unless  the  extra
       argument  is  non-NULL  and  points  to  a pcre_extra block that is the
       result of a successful study with PCRE_STUDY_JIT_COMPILE etc.


JIT STACK FAQ

       (1) Why do we need JIT stacks?

       PCRE (and JIT) is a recursive, depth-first engine, so it needs a  stack
       where  the local data of the current node is pushed before checking its
       child nodes.  Allocating real machine stack on some platforms is diffi-
       cult. For example, the stack chain needs to be updated every time if we
       extend the stack on PowerPC.  Although it  is  possible,  its  updating
       time overhead decreases performance. So we do the recursion in memory.

       (2) Why don't we simply allocate blocks of memory with malloc()?

       Modern  operating  systems  have  a  nice  feature: they can reserve an
       address space instead of allocating memory. We can safely allocate mem-
       ory  pages  inside  this address space, so the stack could grow without
       moving memory data (this is important because of pointers). Thus we can
       allocate  1M  address space, and use only a single memory page (usually
       4K) if that is enough. However, we can still grow up to 1M  anytime  if
       needed.

       (3) Who "owns" a JIT stack?

       The owner of the stack is the user program, not the JIT studied pattern
       or anything else. The user program must ensure that if a stack is  used
       by  pcre_exec(), (that is, it is assigned to the pattern currently run-
       ning), that stack must not be used by any other threads (to avoid over-
       writing the same memory area). The best practice for multithreaded pro-
       grams is to allocate a stack for each thread,  and  return  this  stack
       through the JIT callback function.

       (4) When should a JIT stack be freed?

       You can free a JIT stack at any time, as long as it will not be used by
       pcre_exec() again. When you assign the  stack  to  a  pattern,  only  a
       pointer  is set. There is no reference counting or any other magic. You
       can free the patterns and stacks in any order,  anytime.  Just  do  not
       call  pcre_exec() with a pattern pointing to an already freed stack, as
       that will cause SEGFAULT. (Also, do not free a stack currently used  by
       pcre_exec()  in  another  thread). You can also replace the stack for a
       pattern at any time. You  can  even  free  the  previous  stack  before
       assigning a replacement.

       (5)  Should  I  allocate/free  a  stack every time before/after calling
       pcre_exec()?

       No, because this is too costly in  terms  of  resources.  However,  you
       could  implement  some clever idea which release the stack if it is not
       used in let's say two minutes. The JIT callback can help to achive this
       without keeping a list of the currently JIT studied patterns.

       (6)  OK, the stack is for long term memory allocation. But what happens
       if a pattern causes stack overflow with a stack of 1M? Is that 1M  kept
       until the stack is freed?

       Especially  on embedded sytems, it might be a good idea to release mem-
       ory sometimes without freeing the stack. There is no API  for  this  at
       the  moment.  Probably a function call which returns with the currently
       allocated memory for any stack and another which allows releasing  mem-
       ory (shrinking the stack) would be a good idea if someone needs this.

       (7) This is too much of a headache. Isn't there any better solution for
       JIT stack handling?

       No, thanks to Windows. If POSIX threads were used everywhere, we  could
       throw out this complicated API.


EXAMPLE CODE

       This  is  a  single-threaded example that specifies a JIT stack without
       using a callback.

         int rc;
         int ovector[30];
         pcre *re;
         pcre_extra *extra;
         pcre_jit_stack *jit_stack;

         re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
         /* Check for errors */
         extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
         jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
         /* Check for error (NULL) */
         pcre_assign_jit_stack(extra, NULL, jit_stack);
         rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
         /* Check results */
         pcre_free(re);
         pcre_free_study(extra);
         pcre_jit_stack_free(jit_stack);


SEE ALSO

       pcreapi(3)


AUTHOR

       Philip Hazel (FAQ by Zoltan Herczeg)
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 04 May 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREPARTIAL(3)                                                  PCREPARTIAL(3)


NAME
       PCRE - Perl-compatible regular expressions


PARTIAL MATCHING IN PCRE

       In normal use of PCRE, if the subject string that is passed to a match-
       ing function matches as far as it goes, but is too short to  match  the
       entire pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances
       where it might be helpful to distinguish this case from other cases  in
       which there is no match.

       Consider, for example, an application where a human is required to type
       in data for a field with specific formatting requirements.  An  example
       might be a date in the form ddmmmyy, defined by this pattern:

         ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$

       If the application sees the user's keystrokes one by one, and can check
       that what has been typed so far is potentially valid,  it  is  able  to
       raise  an  error  as  soon  as  a  mistake  is made, by beeping and not
       reflecting the character that has been typed, for example. This immedi-
       ate  feedback is likely to be a better user interface than a check that
       is delayed until the entire string has been entered.  Partial  matching
       can  also be useful when the subject string is very long and is not all
       available at once.

       PCRE supports partial matching by means of  the  PCRE_PARTIAL_SOFT  and
       PCRE_PARTIAL_HARD  options,  which  can  be set when calling any of the
       matching functions. For backwards compatibility, PCRE_PARTIAL is a syn-
       onym  for  PCRE_PARTIAL_SOFT.  The essential difference between the two
       options is whether or not a partial match is preferred to  an  alterna-
       tive complete match, though the details differ between the two types of
       matching function. If both options  are  set,  PCRE_PARTIAL_HARD  takes
       precedence.

       If  you  want to use partial matching with just-in-time optimized code,
       you must call pcre_study() or pcre16_study() with one or both of  these
       options:

         PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
         PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE

       PCRE_STUDY_JIT_COMPILE  should also be set if you are going to run non-
       partial matches on the same pattern. If the appropriate JIT study  mode
       has not been set for a match, the interpretive matching code is used.

       Setting a partial matching option disables two of PCRE's standard opti-
       mizations. PCRE remembers the last literal data unit in a pattern,  and
       abandons  matching  immediately  if  it  is  not present in the subject
       string. This optimization cannot be used  for  a  subject  string  that
       might  match only partially. If the pattern was studied, PCRE knows the
       minimum length of a matching string, and does not  bother  to  run  the
       matching  function  on  shorter strings. This optimization is also dis-
       abled for partial matching.


PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()

       A partial match occurs during a call to  pcre_exec()  or  pcre16_exec()
       when  the end of the subject string is reached successfully, but match-
       ing cannot continue because more characters  are  needed.  However,  at
       least one character in the subject must have been inspected. This char-
       acter need not form part of the final matched string; lookbehind asser-
       tions  and the \K escape sequence provide ways of inspecting characters
       before the start of a matched substring. The requirement for inspecting
       at  least  one  character  exists because an empty string can always be
       matched; without such a restriction there would  always  be  a  partial
       match of an empty string at the end of the subject.

       If  there  are  at least two slots in the offsets vector when a partial
       match is returned, the first slot is set to the offset of the  earliest
       character that was inspected. For convenience, the second offset points
       to the end of the subject so that a substring can easily be identified.

       For the majority of patterns, the first offset identifies the start  of
       the  partially matched string. However, for patterns that contain look-
       behind assertions, or \K, or begin with \b or  \B,  earlier  characters
       have been inspected while carrying out the match. For example:

         /(?<=abc)123/

       This pattern matches "123", but only if it is preceded by "abc". If the
       subject string is "xyzabc12", the offsets after a partial match are for
       the  substring  "abc12",  because  all  these  characters are needed if
       another match is tried with extra characters added to the subject.

       What happens when a partial match is identified depends on which of the
       two partial matching options are set.

   PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()

       If  PCRE_PARTIAL_SOFT  is set when pcre_exec() or pcre16_exec() identi-
       fies a partial match, the partial match  is  remembered,  but  matching
       continues  as  normal, and other alternatives in the pattern are tried.
       If no complete match  can  be  found,  PCRE_ERROR_PARTIAL  is  returned
       instead of PCRE_ERROR_NOMATCH.

       This  option  is "soft" because it prefers a complete match over a par-
       tial match.  All the various matching items in a pattern behave  as  if
       the  subject string is potentially complete. For example, \z, \Z, and $
       match at the end of the subject, as normal, and for \b and \B  the  end
       of the subject is treated as a non-alphanumeric.

       If  there  is more than one partial match, the first one that was found
       provides the data that is returned. Consider this pattern:

         /123\w+X|dogY/

       If this is matched against the subject string "abc123dog", both  alter-
       natives  fail  to  match,  but the end of the subject is reached during
       matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set  to  3
       and  9, identifying "123dog" as the first partial match that was found.
       (In this example, there are two partial matches, because "dog"  on  its
       own partially matches the second alternative.)

   PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()

       If   PCRE_PARTIAL_HARD   is   set  for  pcre_exec()  or  pcre16_exec(),
       PCRE_ERROR_PARTIAL is returned as soon as a  partial  match  is  found,
       without continuing to search for possible complete matches. This option
       is "hard" because it prefers an earlier partial match over a later com-
       plete  match.  For  this reason, the assumption is made that the end of
       the supplied subject string may not be the true end  of  the  available
       data, and so, if \z, \Z, \b, \B, or $ are encountered at the end of the
       subject, the result is PCRE_ERROR_PARTIAL, provided that at  least  one
       character in the subject has been inspected.

       Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16 subject
       strings are checked for validity. Normally, an invalid sequence  causes
       the  error  PCRE_ERROR_BADUTF8  or PCRE_ERROR_BADUTF16. However, in the
       special case of a truncated  character  at  the  end  of  the  subject,
       PCRE_ERROR_SHORTUTF8   or   PCRE_ERROR_SHORTUTF16   is   returned  when
       PCRE_PARTIAL_HARD is set.

   Comparing hard and soft partial matching

       The difference between the two partial matching options can  be  illus-
       trated by a pattern such as:

         /dog(sbody)?/

       This  matches either "dog" or "dogsbody", greedily (that is, it prefers
       the longer string if possible). If it is  matched  against  the  string
       "dog"  with  PCRE_PARTIAL_SOFT,  it  yields a complete match for "dog".
       However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL.
       On  the  other hand, if the pattern is made ungreedy the result is dif-
       ferent:

         /dog(sbody)??/

       In this case the result is always a  complete  match  because  that  is
       found  first,  and  matching  never  continues after finding a complete
       match. It might be easier to follow this explanation by thinking of the
       two patterns like this:

         /dog(sbody)?/    is the same as  /dogsbody|dog/
         /dog(sbody)??/   is the same as  /dog|dogsbody/

       The  second pattern will never match "dogsbody", because it will always
       find the shorter match first.


PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()

       The DFA functions move along the subject string character by character,
       without  backtracking,  searching  for  all possible matches simultane-
       ously. If the end of the subject is reached before the end of the  pat-
       tern,  there is the possibility of a partial match, again provided that
       at least one character has been inspected.

       When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned  only  if
       there  have  been  no complete matches. Otherwise, the complete matches
       are returned.  However, if PCRE_PARTIAL_HARD is set,  a  partial  match
       takes  precedence  over any complete matches. The portion of the string
       that was inspected when the longest partial match was found is  set  as
       the first matching string, provided there are at least two slots in the
       offsets vector.

       Because the DFA functions always search for all possible  matches,  and
       there  is  no  difference between greedy and ungreedy repetition, their
       behaviour is different  from  the  standard  functions  when  PCRE_PAR-
       TIAL_HARD  is  set.  Consider  the  string  "dog"  matched  against the
       ungreedy pattern shown above:

         /dog(sbody)??/

       Whereas the standard functions stop as soon as they find  the  complete
       match  for  "dog",  the  DFA  functions also find the partial match for
       "dogsbody", and so return that when PCRE_PARTIAL_HARD is set.


PARTIAL MATCHING AND WORD BOUNDARIES

       If a pattern ends with one of sequences \b or \B, which test  for  word
       boundaries,  partial  matching with PCRE_PARTIAL_SOFT can give counter-
       intuitive results. Consider this pattern:

         /\bcat\b/

       This matches "cat", provided there is a word boundary at either end. If
       the subject string is "the cat", the comparison of the final "t" with a
       following character cannot take place, so a  partial  match  is  found.
       However,  normal  matching carries on, and \b matches at the end of the
       subject when the last character is a letter, so  a  complete  match  is
       found.   The   result,  therefore,  is  not  PCRE_ERROR_PARTIAL.  Using
       PCRE_PARTIAL_HARD in this case does yield  PCRE_ERROR_PARTIAL,  because
       then the partial match takes precedence.


FORMERLY RESTRICTED PATTERNS

       For releases of PCRE prior to 8.00, because of the way certain internal
       optimizations  were  implemented  in  the  pcre_exec()  function,   the
       PCRE_PARTIAL  option  (predecessor  of  PCRE_PARTIAL_SOFT) could not be
       used with all patterns. From release 8.00 onwards, the restrictions  no
       longer  apply,  and partial matching with can be requested for any pat-
       tern.

       Items that were formerly restricted were repeated single characters and
       repeated  metasequences. If PCRE_PARTIAL was set for a pattern that did
       not conform to the restrictions, pcre_exec() returned  the  error  code
       PCRE_ERROR_BADPARTIAL  (-13).  This error code is no longer in use. The
       PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if  a  compiled
       pattern can be used for partial matching now always returns 1.


EXAMPLE OF PARTIAL MATCHING USING PCRETEST

       If  the  escape  sequence  \P  is  present in a pcretest data line, the
       PCRE_PARTIAL_SOFT option is used for  the  match.  Here  is  a  run  of
       pcretest that uses the date example quoted above:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04\P
          0: 25jun04
          1: jun
         data> 25dec3\P
         Partial match: 23dec3
         data> 3ju\P
         Partial match: 3ju
         data> 3juj\P
         No match
         data> j\P
         No match

       The  first  data  string  is  matched completely, so pcretest shows the
       matched substrings. The remaining four strings do not  match  the  com-
       plete pattern, but the first two are partial matches. Similar output is
       obtained if DFA matching is used.

       If the escape sequence \P is present more than once in a pcretest  data
       line, the PCRE_PARTIAL_HARD option is set for the match.


MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()

       When  a  partial match has been found using a DFA matching function, it
       is possible to continue the match by providing additional subject  data
       and  calling  the function again with the same compiled regular expres-
       sion, this time setting the PCRE_DFA_RESTART option. You must pass  the
       same working space as before, because this is where details of the pre-
       vious partial match are stored. Here  is  an  example  using  pcretest,
       using  the  \R  escape  sequence to set the PCRE_DFA_RESTART option (\D
       specifies the use of the DFA matching function):

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 23ja\P\D
         Partial match: 23ja
         data> n05\R\D
          0: n05

       The first call has "23ja" as the subject, and requests  partial  match-
       ing;  the  second  call  has  "n05"  as  the  subject for the continued
       (restarted) match.  Notice that when the match is  complete,  only  the
       last  part  is  shown;  PCRE  does not retain the previously partially-
       matched string. It is up to the calling program to do that if it  needs
       to.

       You  can  set  the  PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
       PCRE_DFA_RESTART to continue partial matching over  multiple  segments.
       This  facility can be used to pass very long subject strings to the DFA
       matching functions.


MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()

       From release 8.00, the standard matching functions can also be used  to
       do multi-segment matching. Unlike the DFA functions, it is not possible
       to restart the previous match with a new segment of data. Instead,  new
       data must be added to the previous subject string, and the entire match
       re-run, starting from the point where the partial match occurred.  Ear-
       lier data can be discarded.

       It  is best to use PCRE_PARTIAL_HARD in this situation, because it does
       not treat the end of a segment as the end of the subject when  matching
       \z,  \Z,  \b,  \B,  and  $. Consider an unanchored pattern that matches
       dates:

           re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
         data> The date is 23ja\P\P
         Partial match: 23ja

       At this stage, an application could discard the text preceding  "23ja",
       add  on  text  from  the  next  segment, and call the matching function
       again. Unlike the DFA matching functions, the  entire  matching  string
       must  always be available, and the complete matching process occurs for
       each call, so more memory and more processing time is needed.

       Note: If the pattern contains lookbehind assertions, or \K,  or  starts
       with \b or \B, the string that is returned for a partial match includes
       characters that precede the partially matched  string  itself,  because
       these  must be retained when adding on more characters for a subsequent
       matching attempt.  However, in some cases you may need to  retain  even
       earlier characters, as discussed in the next section.


ISSUES WITH MULTI-SEGMENT MATCHING

       Certain types of pattern may give problems with multi-segment matching,
       whichever matching function is used.

       1. If the pattern contains a test for the beginning of a line, you need
       to  pass  the  PCRE_NOTBOL  option when the subject string for any call
       does start at the beginning of a line.  There  is  also  a  PCRE_NOTEOL
       option, but in practice when doing multi-segment matching you should be
       using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL.

       2. Lookbehind assertions that have already been obeyed are catered  for
       in the offsets that are returned for a partial match. However a lookbe-
       hind assertion later in the pattern could require even earlier  charac-
       ters   to  be  inspected.  You  can  handle  this  case  by  using  the
       PCRE_INFO_MAXLOOKBEHIND    option    of    the    pcre_fullinfo()    or
       pcre16_fullinfo() functions to obtain the length of the largest lookbe-
       hind in the pattern. This length is given in characters, not bytes.  If
       you  always  retain  at least that many characters before the partially
       matched string, all should be well. (Of course, near the start  of  the
       subject,  fewer  characters may be present; in that case all characters
       should be retained.)

       3. Because a partial match must always contain at least one  character,
       what  might  be  considered a partial match of an empty string actually
       gives a "no match" result. For example:

           re> /c(?<=abc)x/
         data> ab\P
         No match

       If the next segment begins "cx", a match should be found, but this will
       only  happen  if characters from the previous segment are retained. For
       this reason, a "no match" result  should  be  interpreted  as  "partial
       match of an empty string" when the pattern contains lookbehinds.

       4.  Matching  a subject string that is split into multiple segments may
       not always produce exactly the same result as matching over one  single
       long  string,  especially  when  PCRE_PARTIAL_SOFT is used. The section
       "Partial Matching and Word Boundaries" above describes  an  issue  that
       arises  if  the  pattern ends with \b or \B. Another kind of difference
       may occur when there are multiple matching possibilities, because  (for
       PCRE_PARTIAL_SOFT)  a partial match result is given only when there are
       no completed matches. This means that as soon as the shortest match has
       been  found,  continuation to a new subject segment is no longer possi-
       ble. Consider again this pcretest example:

           re> /dog(sbody)?/
         data> dogsb\P
          0: dog
         data> do\P\D
         Partial match: do
         data> gsb\R\P\D
          0: g
         data> dogsbody\D
          0: dogsbody
          1: dog

       The first data line passes the string "dogsb" to  a  standard  matching
       function,  setting the PCRE_PARTIAL_SOFT option. Although the string is
       a partial match for "dogsbody", the result is  not  PCRE_ERROR_PARTIAL,
       because  the  shorter string "dog" is a complete match. Similarly, when
       the subject is presented to a DFA matching function  in  several  parts
       ("do"  and  "gsb"  being  the first two) the match stops when "dog" has
       been found, and it is not possible to continue.  On the other hand,  if
       "dogsbody"  is  presented  as  a single string, a DFA matching function
       finds both matches.

       Because of these problems, it is best  to  use  PCRE_PARTIAL_HARD  when
       matching  multi-segment  data.  The  example above then behaves differ-
       ently:

           re> /dog(sbody)?/
         data> dogsb\P\P
         Partial match: dogsb
         data> do\P\D
         Partial match: do
         data> gsb\R\P\P\D
         Partial match: gsb

       5. Patterns that contain alternatives at the top level which do not all
       start  with  the  same  pattern  item  may  not  work  as expected when
       PCRE_DFA_RESTART is used. For example, consider this pattern:

         1234|3789

       If the first part of the subject is "ABC123", a partial  match  of  the
       first  alternative  is found at offset 3. There is no partial match for
       the second alternative, because such a match does not start at the same
       point  in  the  subject  string. Attempting to continue with the string
       "7890" does not yield a match  because  only  those  alternatives  that
       match  at  one  point in the subject are remembered. The problem arises
       because the start of the second alternative matches  within  the  first
       alternative.  There  is  no  problem with anchored patterns or patterns
       such as:

         1234|ABCD

       where no string can be a partial match for both alternatives.  This  is
       not  a  problem  if  a  standard matching function is used, because the
       entire match has to be rerun each time:

           re> /1234|3789/
         data> ABC123\P\P
         Partial match: 123
         data> 1237890
          0: 3789

       Of course, instead of using PCRE_DFA_RESTART, the same technique of re-
       running  the  entire match can also be used with the DFA matching func-
       tions. Another possibility is to work with two buffers.  If  a  partial
       match  at  offset  n in the first buffer is followed by "no match" when
       PCRE_DFA_RESTART is used on the second buffer, you can then try  a  new
       match starting at offset n+1 in the first buffer.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 24 February 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREPRECOMPILE(3)                                            PCREPRECOMPILE(3)


NAME
       PCRE - Perl-compatible regular expressions


SAVING AND RE-USING PRECOMPILED PCRE PATTERNS

       If  you  are running an application that uses a large number of regular
       expression patterns, it may be useful to store them  in  a  precompiled
       form  instead  of  having to compile them every time the application is
       run.  If you are not  using  any  private  character  tables  (see  the
       pcre_maketables()  documentation),  this is relatively straightforward.
       If you are using private tables, it is a little bit  more  complicated.
       However,  if you are using the just-in-time optimization feature, it is
       not possible to save and reload the JIT data.

       If you save compiled patterns to a file, you can copy them to a differ-
       ent host and run them there. If the two hosts have different endianness
       (byte order), you should run the  pcre[16]_pattern_to_host_byte_order()
       function on the new host before trying to match the pattern. The match-
       ing functions return PCRE_ERROR_BADENDIANNESS if they detect a  pattern
       with the wrong endianness.

       Compiling  regular  expressions with one version of PCRE for use with a
       different version is not guaranteed to work and may cause crashes,  and
       saving  and  restoring  a  compiled  pattern loses any JIT optimization
       data.


SAVING A COMPILED PATTERN

       The value returned by pcre[16]_compile() points to a  single  block  of
       memory  that  holds  the  compiled pattern and associated data. You can
       find the length of this block in bytes by  calling  pcre[16]_fullinfo()
       with  an  argument of PCRE_INFO_SIZE. You can then save the data in any
       appropriate manner. Here is sample code for the 8-bit library that com-
       piles  a  pattern and writes it to a file. It assumes that the variable
       fd refers to a file that is open for output:

         int erroroffset, rc, size;
         char *error;
         pcre *re;

         re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
         if (re == NULL) { ... handle errors ... }
         rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
         if (rc < 0) { ... handle errors ... }
         rc = fwrite(re, 1, size, fd);
         if (rc != size) { ... handle errors ... }

       In this example, the bytes  that  comprise  the  compiled  pattern  are
       copied  exactly.  Note that this is binary data that may contain any of
       the 256 possible byte  values.  On  systems  that  make  a  distinction
       between binary and non-binary data, be sure that the file is opened for
       binary output.

       If you want to write more than one pattern to a file, you will have  to
       devise  a  way of separating them. For binary data, preceding each pat-
       tern with its length is probably  the  most  straightforward  approach.
       Another  possibility is to write out the data in hexadecimal instead of
       binary, one pattern to a line.

       Saving compiled patterns in a file is only one possible way of  storing
       them  for later use. They could equally well be saved in a database, or
       in the memory of some daemon process that passes them  via  sockets  to
       the processes that want them.

       If the pattern has been studied, it is also possible to save the normal
       study data in a similar way to the compiled pattern itself. However, if
       the PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is cre-
       ated cannot be saved because it is too dependent on the  current  envi-
       ronment.    When    studying    generates    additional    information,
       pcre[16]_study() returns a pointer to a pcre[16]_extra data block.  Its
       format  is  defined in the section on matching a pattern in the pcreapi
       documentation. The study_data field points to the  binary  study  data,
       and  this  is what you must save (not the pcre[16]_extra block itself).
       The  length  of  the  study   data   can   be   obtained   by   calling
       pcre[16]_fullinfo()  with  an argument of PCRE_INFO_STUDYSIZE. Remember
       to check that pcre[16]_study() did return a non-NULL value before  try-
       ing to save the study data.


RE-USING A PRECOMPILED PATTERN

       Re-using  a  precompiled pattern is straightforward. Having reloaded it
       into main memory, called pcre[16]_pattern_to_host_byte_order() if  nec-
       essary,  you pass its pointer to pcre[16]_exec() or pcre[16]_dfa_exec()
       in the usual way.

       However, if you passed a pointer to custom character  tables  when  the
       pattern was compiled (the tableptr argument of pcre[16]_compile()), you
       must   now   pass   a   similar   pointer   to    pcre[16]_exec()    or
       pcre[16]_dfa_exec(),  because the value saved with the compiled pattern
       will obviously be nonsense. A field in a pcre[16]_extra() block is used
       to pass this data, as described in the section on matching a pattern in
       the pcreapi documentation.

       If you did not provide custom character tables  when  the  pattern  was
       compiled, the pointer in the compiled pattern is NULL, which causes the
       matching functions to use PCRE's internal tables. Thus, you do not need
       to take any special action at run time in this case.

       If  you  saved study data with the compiled pattern, you need to create
       your own pcre[16]_extra data block and  set  the  study_data  field  to
       point   to   the   reloaded   study   data.   You  must  also  set  the
       PCRE_EXTRA_STUDY_DATA bit in the flags field  to  indicate  that  study
       data  is  present.  Then  pass the pcre[16]_extra block to the matching
       function in the usual way. If the pattern was studied for  just-in-time
       optimization,  that  data  cannot  be  saved,  and  so  is  lost  by  a
       save/restore cycle.


COMPATIBILITY WITH DIFFERENT PCRE RELEASES

       In general, it is safest to  recompile  all  saved  patterns  when  you
       update  to  a new PCRE release, though not all updates actually require
       this.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 10 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREPERFORM(3)                                                  PCREPERFORM(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE PERFORMANCE

       Two  aspects  of performance are discussed below: memory usage and pro-
       cessing time. The way you express your pattern as a regular  expression
       can affect both of them.


COMPILED PATTERN MEMORY USAGE

       Patterns  are compiled by PCRE into a reasonably efficient interpretive
       code, so that most simple patterns do not  use  much  memory.  However,
       there  is  one case where the memory usage of a compiled pattern can be
       unexpectedly large. If a parenthesized subpattern has a quantifier with
       a minimum greater than 1 and/or a limited maximum, the whole subpattern
       is repeated in the compiled code. For example, the pattern

         (abc|def){2,4}

       is compiled as if it were

         (abc|def)(abc|def)((abc|def)(abc|def)?)?

       (Technical aside: It is done this way so that backtrack  points  within
       each of the repetitions can be independently maintained.)

       For  regular expressions whose quantifiers use only small numbers, this
       is not usually a problem. However, if the numbers are large,  and  par-
       ticularly  if  such repetitions are nested, the memory usage can become
       an embarrassment. For example, the very simple pattern

         ((ab){1,1000}c){1,3}

       uses 51K bytes when compiled using the 8-bit library. When PCRE is com-
       piled  with  its  default  internal pointer size of two bytes, the size
       limit on a compiled pattern is 64K data units, and this is reached with
       the  above  pattern  if  the outer repetition is increased from 3 to 4.
       PCRE can be compiled to use larger internal pointers  and  thus  handle
       larger  compiled patterns, but it is better to try to rewrite your pat-
       tern to use less memory if you can.

       One way of reducing the memory usage for such patterns is to  make  use
       of PCRE's "subroutine" facility. Re-writing the above pattern as

         ((ab)(?2){0,999}c)(?1){0,2}

       reduces the memory requirements to 18K, and indeed it remains under 20K
       even with the outer repetition increased to 100. However, this  pattern
       is  not  exactly equivalent, because the "subroutine" calls are treated
       as atomic groups into which there can be no backtracking if there is  a
       subsequent  matching  failure.  Therefore,  PCRE cannot do this kind of
       rewriting automatically.  Furthermore, there is a  noticeable  loss  of
       speed  when executing the modified pattern. Nevertheless, if the atomic
       grouping is not a problem and the loss of  speed  is  acceptable,  this
       kind  of  rewriting will allow you to process patterns that PCRE cannot
       otherwise handle.


STACK USAGE AT RUN TIME

       When pcre_exec() or pcre16_exec() is used for matching,  certain  kinds
       of  pattern  can cause it to use large amounts of the process stack. In
       some environments the default process stack is quite small, and  if  it
       runs  out  the result is often SIGSEGV. This issue is probably the most
       frequently raised problem with PCRE. Rewriting your pattern  can  often
       help. The pcrestack documentation discusses this issue in detail.


PROCESSING TIME

       Certain  items  in regular expression patterns are processed more effi-
       ciently than others. It is more efficient to use a character class like
       [aeiou]   than   a   set   of  single-character  alternatives  such  as
       (a|e|i|o|u). In general, the simplest construction  that  provides  the
       required behaviour is usually the most efficient. Jeffrey Friedl's book
       contains a lot of useful general discussion  about  optimizing  regular
       expressions  for  efficient  performance.  This document contains a few
       observations about PCRE.

       Using Unicode character properties (the \p,  \P,  and  \X  escapes)  is
       slow,  because PCRE has to scan a structure that contains data for over
       fifteen thousand characters whenever it needs a  character's  property.
       If  you  can  find  an  alternative pattern that does not use character
       properties, it will probably be faster.

       By default, the escape sequences \b, \d, \s,  and  \w,  and  the  POSIX
       character  classes  such  as  [:alpha:]  do not use Unicode properties,
       partly for backwards compatibility, and partly for performance reasons.
       However,  you can set PCRE_UCP if you want Unicode character properties
       to be used. This can double the matching time for  items  such  as  \d,
       when matched with a traditional matching function; the performance loss
       is less with a DFA matching function, and in both cases  there  is  not
       much difference for \b.

       When  a  pattern  begins  with .* not in parentheses, or in parentheses
       that are not the subject of a backreference, and the PCRE_DOTALL option
       is  set, the pattern is implicitly anchored by PCRE, since it can match
       only at the start of a subject string. However, if PCRE_DOTALL  is  not
       set,  PCRE  cannot  make this optimization, because the . metacharacter
       does not then match a newline, and if the subject string contains  new-
       lines,  the  pattern may match from the character immediately following
       one of them instead of from the very start. For example, the pattern

         .*second

       matches the subject "first\nand second" (where \n stands for a  newline
       character),  with the match starting at the seventh character. In order
       to do this, PCRE has to retry the match starting after every newline in
       the subject.

       If  you  are using such a pattern with subject strings that do not con-
       tain newlines, the best performance is obtained by setting PCRE_DOTALL,
       or  starting  the pattern with ^.* or ^.*? to indicate explicit anchor-
       ing. That saves PCRE from having to scan along the subject looking  for
       a newline to restart at.

       Beware  of  patterns  that contain nested indefinite repeats. These can
       take a long time to run when applied to a string that does  not  match.
       Consider the pattern fragment

         ^(a+)*

       This  can  match "aaaa" in 16 different ways, and this number increases
       very rapidly as the string gets longer. (The * repeat can match  0,  1,
       2,  3, or 4 times, and for each of those cases other than 0 or 4, the +
       repeats can match different numbers of times.) When  the  remainder  of
       the pattern is such that the entire match is going to fail, PCRE has in
       principle to try  every  possible  variation,  and  this  can  take  an
       extremely long time, even for relatively short strings.

       An optimization catches some of the more simple cases such as

         (a+)*b

       where  a  literal  character  follows. Before embarking on the standard
       matching procedure, PCRE checks that there is a "b" later in  the  sub-
       ject  string, and if there is not, it fails the match immediately. How-
       ever, when there is no following literal this  optimization  cannot  be
       used. You can see the difference by comparing the behaviour of

         (a+)*\d

       with  the  pattern  above.  The former gives a failure almost instantly
       when applied to a whole line of  "a"  characters,  whereas  the  latter
       takes an appreciable time with strings longer than about 20 characters.

       In many cases, the solution to this kind of performance issue is to use
       an atomic group or a possessive quantifier.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 09 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREPOSIX(3)                                                      PCREPOSIX(3)


NAME
       PCRE - Perl-compatible regular expressions.


SYNOPSIS OF POSIX API

       #include <pcreposix.h>

       int regcomp(regex_t *preg, const char *pattern,
            int cflags);

       int regexec(regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

       size_t regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

       void regfree(regex_t *preg);


DESCRIPTION

       This  set  of functions provides a POSIX-style API for the PCRE regular
       expression 8-bit library. See the pcreapi documentation for a  descrip-
       tion  of  PCRE's native API, which contains much additional functional-
       ity. There is no POSIX-style wrapper for PCRE's 16-bit library.

       The functions described here are just wrapper functions that ultimately
       call  the  PCRE  native  API.  Their  prototypes  are  defined  in  the
       pcreposix.h header file, and on Unix  systems  the  library  itself  is
       called  pcreposix.a,  so  can  be accessed by adding -lpcreposix to the
       command for linking an application that uses them.  Because  the  POSIX
       functions call the native ones, it is also necessary to add -lpcre.

       I  have implemented only those POSIX option bits that can be reasonably
       mapped to PCRE native options. In addition, the option REG_EXTENDED  is
       defined  with  the  value  zero. This has no effect, but since programs
       that are written to the POSIX interface often use  it,  this  makes  it
       easier  to  slot  in PCRE as a replacement library. Other POSIX options
       are not even defined.

       There are also some other options that are not defined by POSIX.  These
       have been added at the request of users who want to make use of certain
       PCRE-specific features via the POSIX calling interface.

       When PCRE is called via these functions, it is only  the  API  that  is
       POSIX-like  in  style.  The syntax and semantics of the regular expres-
       sions themselves are still those of Perl, subject  to  the  setting  of
       various  PCRE  options, as described below. "POSIX-like in style" means
       that the API approximates to the POSIX  definition;  it  is  not  fully
       POSIX-compatible,  and  in  multi-byte  encoding domains it is probably
       even less compatible.

       The header for these functions is supplied as pcreposix.h to avoid  any
       potential  clash  with  other  POSIX  libraries.  It can, of course, be
       renamed or aliased as regex.h, which is the "correct" name. It provides
       two  structure  types,  regex_t  for  compiled internal forms, and reg-
       match_t for returning captured substrings. It also  defines  some  con-
       stants  whose  names  start  with  "REG_";  these  are used for setting
       options and identifying error codes.


COMPILING A PATTERN

       The function regcomp() is called to compile a pattern into an  internal
       form.  The  pattern  is  a C string terminated by a binary zero, and is
       passed in the argument pattern. The preg argument is  a  pointer  to  a
       regex_t  structure that is used as a base for storing information about
       the compiled regular expression.

       The argument cflags is either zero, or contains one or more of the bits
       defined by the following macros:

         REG_DOTALL

       The PCRE_DOTALL option is set when the regular expression is passed for
       compilation to the native function. Note that REG_DOTALL is not part of
       the POSIX standard.

         REG_ICASE

       The  PCRE_CASELESS  option is set when the regular expression is passed
       for compilation to the native function.

         REG_NEWLINE

       The PCRE_MULTILINE option is set when the regular expression is  passed
       for  compilation  to the native function. Note that this does not mimic
       the defined POSIX behaviour for REG_NEWLINE  (see  the  following  sec-
       tion).

         REG_NOSUB

       The  PCRE_NO_AUTO_CAPTURE  option is set when the regular expression is
       passed for compilation to the native function. In addition, when a pat-
       tern  that is compiled with this flag is passed to regexec() for match-
       ing, the nmatch and pmatch  arguments  are  ignored,  and  no  captured
       strings are returned.

         REG_UCP

       The  PCRE_UCP  option  is set when the regular expression is passed for
       compilation to the native function. This causes  PCRE  to  use  Unicode
       properties  when  matchine  \d,  \w,  etc., instead of just recognizing
       ASCII values. Note that REG_UTF8 is not part of the POSIX standard.

         REG_UNGREEDY

       The PCRE_UNGREEDY option is set when the regular expression  is  passed
       for  compilation  to the native function. Note that REG_UNGREEDY is not
       part of the POSIX standard.

         REG_UTF8

       The PCRE_UTF8 option is set when the regular expression is  passed  for
       compilation  to the native function. This causes the pattern itself and
       all data strings used for matching it to be treated as  UTF-8  strings.
       Note that REG_UTF8 is not part of the POSIX standard.

       In  the  absence  of  these  flags, no options are passed to the native
       function.  This means the the  regex  is  compiled  with  PCRE  default
       semantics.  In particular, the way it handles newline characters in the
       subject string is the Perl way, not the POSIX way.  Note  that  setting
       PCRE_MULTILINE  has only some of the effects specified for REG_NEWLINE.
       It does not affect the way newlines are matched by . (they are not)  or
       by a negative class such as [^a] (they are).

       The  yield of regcomp() is zero on success, and non-zero otherwise. The
       preg structure is filled in on success, and one member of the structure
       is  public: re_nsub contains the number of capturing subpatterns in the
       regular expression. Various error codes are defined in the header file.

       NOTE: If the yield of regcomp() is non-zero, you must  not  attempt  to
       use the contents of the preg structure. If, for example, you pass it to
       regexec(), the result is undefined and your program is likely to crash.


MATCHING NEWLINE CHARACTERS

       This area is not simple, because POSIX and Perl take different views of
       things.   It  is  not possible to get PCRE to obey POSIX semantics, but
       then PCRE was never intended to be a POSIX engine. The following  table
       lists  the  different  possibilities for matching newline characters in
       PCRE:

                                 Default   Change with

         . matches newline          no     PCRE_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE_DOLLARENDONLY
         $ matches \n in middle     no     PCRE_MULTILINE
         ^ matches \n in middle     no     PCRE_MULTILINE

       This is the equivalent table for POSIX:

                                 Default   Change with

         . matches newline          yes    REG_NEWLINE
         newline matches [^a]       yes    REG_NEWLINE
         $ matches \n at end        no     REG_NEWLINE
         $ matches \n in middle     no     REG_NEWLINE
         ^ matches \n in middle     no     REG_NEWLINE

       PCRE's behaviour is the same as Perl's, except that there is no equiva-
       lent  for  PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is
       no way to stop newline from matching [^a].

       The  default  POSIX  newline  handling  can  be  obtained  by   setting
       PCRE_DOTALL  and  PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
       behave exactly as for the REG_NEWLINE action.


MATCHING A PATTERN

       The function regexec() is called  to  match  a  compiled  pattern  preg
       against  a  given string, which is by default terminated by a zero byte
       (but see REG_STARTEND below), subject to the options in  eflags.  These
       can be:

         REG_NOTBOL

       The PCRE_NOTBOL option is set when calling the underlying PCRE matching
       function.

         REG_NOTEMPTY

       The PCRE_NOTEMPTY option is set when calling the underlying PCRE match-
       ing function. Note that REG_NOTEMPTY is not part of the POSIX standard.
       However, setting this option can give more POSIX-like behaviour in some
       situations.

         REG_NOTEOL

       The PCRE_NOTEOL option is set when calling the underlying PCRE matching
       function.

         REG_STARTEND

       The string is considered to start at string +  pmatch[0].rm_so  and  to
       have  a terminating NUL located at string + pmatch[0].rm_eo (there need
       not actually be a NUL at that location), regardless  of  the  value  of
       nmatch.  This  is a BSD extension, compatible with but not specified by
       IEEE Standard 1003.2 (POSIX.2), and should  be  used  with  caution  in
       software intended to be portable to other systems. Note that a non-zero
       rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location
       of the string, not how it is matched.

       If  the pattern was compiled with the REG_NOSUB flag, no data about any
       matched strings  is  returned.  The  nmatch  and  pmatch  arguments  of
       regexec() are ignored.

       If the value of nmatch is zero, or if the value pmatch is NULL, no data
       about any matched strings is returned.

       Otherwise,the portion of the string that was matched, and also any cap-
       tured substrings, are returned via the pmatch argument, which points to
       an array of nmatch structures of type regmatch_t, containing  the  mem-
       bers  rm_so  and rm_eo. These contain the offset to the first character
       of each substring and the offset to the first character after  the  end
       of  each substring, respectively. The 0th element of the vector relates
       to the entire portion of string that was matched;  subsequent  elements
       relate  to  the capturing subpatterns of the regular expression. Unused
       entries in the array have both structure members set to -1.

       A successful match yields  a  zero  return;  various  error  codes  are
       defined  in  the  header  file,  of which REG_NOMATCH is the "expected"
       failure code.


ERROR MESSAGES

       The regerror() function maps a non-zero errorcode from either regcomp()
       or  regexec()  to  a  printable message. If preg is not NULL, the error
       should have arisen from the use of that structure. A message terminated
       by  a  binary  zero  is  placed  in  errbuf. The length of the message,
       including the zero, is limited to errbuf_size. The yield of  the  func-
       tion is the size of buffer needed to hold the whole message.


MEMORY USAGE

       Compiling  a regular expression causes memory to be allocated and asso-
       ciated with the preg structure. The function regfree() frees  all  such
       memory,  after  which  preg may no longer be used as a compiled expres-
       sion.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 09 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRECPP(3)                                                          PCRECPP(3)


NAME
       PCRE - Perl-compatible regular expressions.


SYNOPSIS OF C++ WRAPPER

       #include <pcrecpp.h>


DESCRIPTION

       The  C++  wrapper  for PCRE was provided by Google Inc. Some additional
       functionality was added by Giuseppe Maxia. This brief man page was con-
       structed  from  the  notes  in the pcrecpp.h file, which should be con-
       sulted for further details. Note that the C++ wrapper supports only the
       original 8-bit PCRE library. There is no 16-bit support at present.


MATCHING INTERFACE

       The  "FullMatch" operation checks that supplied text matches a supplied
       pattern exactly. If pointer arguments are supplied, it  copies  matched
       sub-strings that match sub-patterns into them.

         Example: successful match
            pcrecpp::RE re("h.*o");
            re.FullMatch("hello");

         Example: unsuccessful match (requires full match):
            pcrecpp::RE re("e");
            !re.FullMatch("hello");

         Example: creating a temporary RE object:
            pcrecpp::RE("h.*o").FullMatch("hello");

       You  can pass in a "const char*" or a "string" for "text". The examples
       below tend to use a const char*. You can, as in the different  examples
       above,  store the RE object explicitly in a variable or use a temporary
       RE object. The examples below use one mode or  the  other  arbitrarily.
       Either could correctly be used for any of these examples.

       You must supply extra pointer arguments to extract matched subpieces.

         Example: extracts "ruby" into "s" and 1234 into "i"
            int i;
            string s;
            pcrecpp::RE re("(\\w+):(\\d+)");
            re.FullMatch("ruby:1234", &s, &i);

         Example: does not try to extract any extra sub-patterns
            re.FullMatch("ruby:1234", &s);

         Example: does not try to extract into NULL
            re.FullMatch("ruby:1234", NULL, &i);

         Example: integer overflow causes failure
            !re.FullMatch("ruby:1234567891234", NULL, &i);

         Example: fails because there aren't enough sub-patterns:
            !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);

         Example: fails because string cannot be stored in integer
            !pcrecpp::RE("(.*)").FullMatch("ruby", &i);

       The  provided  pointer  arguments can be pointers to any scalar numeric
       type, or one of:

          string        (matched piece is copied to string)
          StringPiece   (StringPiece is mutated to point to matched piece)
          T             (where "bool T::ParseFrom(const char*, int)" exists)
          NULL          (the corresponding matched sub-pattern is not copied)

       The function returns true iff all of the following conditions are  sat-
       isfied:

         a. "text" matches "pattern" exactly;

         b. The number of matched sub-patterns is >= number of supplied
            pointers;

         c. The "i"th argument has a suitable type for holding the
            string captured as the "i"th sub-pattern. If you pass in
            void * NULL for the "i"th argument, or a non-void * NULL
            of the correct type, or pass fewer arguments than the
            number of sub-patterns, "i"th captured sub-pattern is
            ignored.

       CAVEAT:  An  optional  sub-pattern  that  does not exist in the matched
       string is assigned the empty  string.  Therefore,  the  following  will
       return false (because the empty string is not a valid number):

          int number;
          pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number);

       The  matching interface supports at most 16 arguments per call.  If you
       need   more,   consider    using    the    more    general    interface
       pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.

       NOTE:  Do not use no_arg, which is used internally to mark the end of a
       list of optional arguments, as a placeholder for missing arguments,  as
       this can lead to segfaults.


QUOTING METACHARACTERS

       You  can use the "QuoteMeta" operation to insert backslashes before all
       potentially meaningful characters in a  string.  The  returned  string,
       used as a regular expression, will exactly match the original string.

         Example:
            string quoted = RE::QuoteMeta(unquoted);

       Note  that  it's  legal to escape a character even if it has no special
       meaning in a regular expression -- so this function  does  that.  (This
       also  makes  it  identical  to  the perl function of the same name; see
       "perldoc   -f   quotemeta".)    For   example,    "1.5-2.0?"    becomes
       "1\.5\-2\.0\?".


PARTIAL MATCHES

       You  can  use the "PartialMatch" operation when you want the pattern to
       match any substring of the text.

         Example: simple search for a string:
            pcrecpp::RE("ell").PartialMatch("hello");

         Example: find first number in a string:
            int number;
            pcrecpp::RE re("(\\d+)");
            re.PartialMatch("x*100 + 20", &number);
            assert(number == 100);


UTF-8 AND THE MATCHING INTERFACE

       By default, pattern and text are plain text, one  byte  per  character.
       The  UTF8  flag,  passed  to  the  constructor, causes both pattern and
       string to be treated as UTF-8 text, still a byte stream but potentially
       multiple  bytes  per character. In practice, the text is likelier to be
       UTF-8 than the pattern, but the match returned may depend on  the  UTF8
       flag,  so  always use it when matching UTF8 text. For example, "." will
       match one byte normally but with UTF8 set may match up to  three  bytes
       of a multi-byte character.

         Example:
            pcrecpp::RE_Options options;
            options.set_utf8();
            pcrecpp::RE re(utf8_pattern, options);
            re.FullMatch(utf8_string);

         Example: using the convenience function UTF8():
            pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
            re.FullMatch(utf8_string);

       NOTE: The UTF8 flag is ignored if pcre was not configured with the
             --enable-utf8 flag.


PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE

       PCRE  defines  some  modifiers  to  change  the behavior of the regular
       expression  engine.  The  C++  wrapper  defines  an  auxiliary   class,
       RE_Options,  as  a  vehicle  to pass such modifiers to a RE class. Cur-
       rently, the following modifiers are supported:

          modifier              description               Perl corresponding

          PCRE_CASELESS         case insensitive match      /i
          PCRE_MULTILINE        multiple lines match        /m
          PCRE_DOTALL           dot matches newlines        /s
          PCRE_DOLLAR_ENDONLY   $ matches only at end       N/A
          PCRE_EXTRA            strict escape parsing       N/A
          PCRE_EXTENDED         ignore white spaces         /x
          PCRE_UTF8             handles UTF8 chars          built-in
          PCRE_UNGREEDY         reverses * and *?           N/A
          PCRE_NO_AUTO_CAPTURE  disables capturing parens   N/A (*)

       (*) Both Perl and PCRE allow non capturing parentheses by means of  the
       "?:"  modifier  within the pattern itself. e.g. (?:ab|cd) does not cap-
       ture, while (ab|cd) does.

       For a full account on how each modifier works, please  check  the  PCRE
       API reference page.

       For  each  modifier,  there are two member functions whose name is made
       out of the modifier in  lowercase,  without  the  "PCRE_"  prefix.  For
       instance, PCRE_CASELESS is handled by

         bool caseless()

       which returns true if the modifier is set, and

         RE_Options & set_caseless(bool)

       which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can
       be accessed through  the  set_match_limit()  and  match_limit()  member
       functions.  Setting match_limit to a non-zero value will limit the exe-
       cution of pcre to keep it from doing bad things like blowing the  stack
       or  taking  an  eternity  to  return  a result. A value of 5000 is good
       enough to stop stack blowup in a 2MB thread stack. Setting  match_limit
       to   zero   disables   match  limiting.  Alternatively,  you  can  call
       match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION  to
       limit  how  much  PCRE  recurses.  match_limit()  limits  the number of
       matches PCRE does; match_limit_recursion() limits the depth of internal
       recursion, and therefore the amount of stack that is used.

       Normally,  to  pass  one or more modifiers to a RE class, you declare a
       RE_Options object, set the appropriate options, and pass this object to
       a RE constructor. Example:

          RE_Options opt;
          opt.set_caseless(true);
          if (RE("HELLO", opt).PartialMatch("hello world")) ...

       RE_options has two constructors. The default constructor takes no argu-
       ments and creates a set of flags that are off by default. The  optional
       parameter  option_flags is to facilitate transfer of legacy code from C
       programs.  This lets you do

          RE(pattern,
            RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str);

       However, new code is better off doing

          RE(pattern,
            RE_Options().set_caseless(true).set_multiline(true))
              .PartialMatch(str);

       If you are going to pass one of the most used modifiers, there are some
       convenience functions that return a RE_Options class with the appropri-
       ate modifier already set: CASELESS(),  UTF8(),  MULTILINE(),  DOTALL(),
       and EXTENDED().

       If  you  need  to set several options at once, and you don't want to go
       through the pains of declaring a RE_Options object and setting  several
       options,  there  is a parallel method that give you such ability on the
       fly. You can concatenate several set_xxxxx()  member  functions,  since
       each  of  them returns a reference to its class object. For example, to
       pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with  one
       statement, you may write:

          RE(" ^ xyz \\s+ .* blah$",
            RE_Options()
              .set_caseless(true)
              .set_extended(true)
              .set_multiline(true)).PartialMatch(sometext);


SCANNING TEXT INCREMENTALLY

       The  "Consume"  operation may be useful if you want to repeatedly match
       regular expressions at the front of a string and skip over them as they
       match.  This requires use of the "StringPiece" type, which represents a
       sub-range of a real string. Like RE,  StringPiece  is  defined  in  the
       pcrecpp namespace.

         Example: read lines of the form "var = value" from a string.
            string contents = ...;                 // Fill string somehow
            pcrecpp::StringPiece input(contents);  // Wrap in a StringPiece

            string var;
            int value;
            pcrecpp::RE re("(\\w+) = (\\d+)\n");
            while (re.Consume(&input, &var, &value)) {
              ...;
            }

       Each  successful  call  to  "Consume"  will  set  "var/value", and also
       advance "input" so it points past the matched text.

       The "FindAndConsume" operation is similar to  "Consume"  but  does  not
       anchor  your  match  at  the  beginning of the string. For example, you
       could extract all words from a string by repeatedly calling

         pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word)


PARSING HEX/OCTAL/C-RADIX NUMBERS

       By default, if you pass a pointer to a numeric value, the corresponding
       text  is  interpreted  as  a  base-10  number. You can instead wrap the
       pointer with a call to one of the operators Hex(), Octal(), or CRadix()
       to  interpret  the text in another base. The CRadix operator interprets
       C-style "0" (base-8) and  "0x"  (base-16)  prefixes,  but  defaults  to
       base-10.

         Example:
           int a, b, c, d;
           pcrecpp::RE re("(.*) (.*) (.*) (.*)");
           re.FullMatch("100 40 0100 0x40",
                        pcrecpp::Octal(&a), pcrecpp::Hex(&b),
                        pcrecpp::CRadix(&c), pcrecpp::CRadix(&d));

       will leave 64 in a, b, c, and d.


REPLACING PARTS OF STRINGS

       You  can  replace the first match of "pattern" in "str" with "rewrite".
       Within "rewrite", backslash-escaped digits (\1 to \9) can  be  used  to
       insert  text  matching  corresponding parenthesized group from the pat-
       tern. \0 in "rewrite" refers to the entire matching text. For example:

         string s = "yabba dabba doo";
         pcrecpp::RE("b+").Replace("d", &s);

       will leave "s" containing "yada dabba doo". The result is true  if  the
       pattern matches and a replacement occurs, false otherwise.

       GlobalReplace  is  like Replace except that it replaces all occurrences
       of the pattern in the string with the  rewrite.  Replacements  are  not
       subject to re-matching. For example:

         string s = "yabba dabba doo";
         pcrecpp::RE("b+").GlobalReplace("d", &s);

       will  leave  "s"  containing  "yada dada doo". It returns the number of
       replacements made.

       Extract is like Replace, except that if the pattern matches,  "rewrite"
       is  copied into "out" (an additional argument) with substitutions.  The
       non-matching portions of "text" are ignored. Returns true iff  a  match
       occurred and the extraction happened successfully;  if no match occurs,
       the string is left unaffected.


AUTHOR

       The C++ wrapper was contributed by Google Inc.
       Copyright (c) 2007 Google Inc.


REVISION

       Last updated: 08 January 2012
------------------------------------------------------------------------------


PCRESAMPLE(3)                                                    PCRESAMPLE(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE SAMPLE PROGRAM

       A simple, complete demonstration program, to get you started with using
       PCRE, is supplied in the file pcredemo.c in the  PCRE  distribution.  A
       listing  of this program is given in the pcredemo documentation. If you
       do not have a copy of the PCRE distribution, you can save this  listing
       to re-create pcredemo.c.

       The  demonstration program, which uses the original PCRE 8-bit library,
       compiles the regular expression that is its first argument, and matches
       it  against  the subject string in its second argument. No PCRE options
       are set, and default character tables are used. If  matching  succeeds,
       the  program  outputs the portion of the subject that matched, together
       with the contents of any captured substrings.

       If the -g option is given on the command line, the program then goes on
       to check for further matches of the same regular expression in the same
       subject string. The logic is a little bit tricky because of the  possi-
       bility  of  matching an empty string. Comments in the code explain what
       is going on.

       If PCRE is installed in the standard include  and  library  directories
       for your operating system, you should be able to compile the demonstra-
       tion program using this command:

         gcc -o pcredemo pcredemo.c -lpcre

       If PCRE is installed elsewhere, you may need to add additional  options
       to  the  command line. For example, on a Unix-like system that has PCRE
       installed in /usr/local, you  can  compile  the  demonstration  program
       using a command like this:

         gcc -o pcredemo -I/usr/local/include pcredemo.c \
             -L/usr/local/lib -lpcre

       In  a  Windows  environment, if you want to statically link the program
       against a non-dll pcre.a file, you must uncomment the line that defines
       PCRE_STATIC  before  including  pcre.h, because otherwise the pcre_mal-
       loc()   and   pcre_free()   exported   functions   will   be   declared
       __declspec(dllimport), with unwanted results.

       Once  you  have  compiled and linked the demonstration program, you can
       run simple tests like this:

         ./pcredemo 'cat|dog' 'the cat sat on the mat'
         ./pcredemo -g 'cat|dog' 'the dog sat on the cat'

       Note that there is a  much  more  comprehensive  test  program,  called
       pcretest,  which  supports  many  more  facilities  for testing regular
       expressions and both PCRE libraries. The pcredemo program  is  provided
       as a simple coding example.

       If  you  try to run pcredemo when PCRE is not installed in the standard
       library directory, you may get an error like  this  on  some  operating
       systems (e.g. Solaris):

         ld.so.1:  a.out:  fatal:  libpcre.so.0:  open failed: No such file or
       directory

       This is caused by the way shared library support works  on  those  sys-
       tems. You need to add

         -R/usr/local/lib

       (for example) to the compile command to get round this problem.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 10 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------
PCRELIMITS(3)                                                    PCRELIMITS(3)


NAME
       PCRE - Perl-compatible regular expressions


SIZE AND OTHER LIMITATIONS

       There  are some size limitations in PCRE but it is hoped that they will
       never in practice be relevant.

       The maximum length of a compiled  pattern  is  approximately  64K  data
       units  (bytes  for  the  8-bit  library,  16-bit  units  for the 16-bit
       library) if PCRE is compiled with the default internal linkage size  of
       2  bytes.  If  you  want  to process regular expressions that are truly
       enormous, you can compile PCRE with an internal linkage size of 3 or  4
       (when  building  the  16-bit  library,  3  is rounded up to 4). See the
       README file in the source distribution and the pcrebuild  documentation
       for  details.  In  these cases the limit is substantially larger.  How-
       ever, the speed of execution is slower.

       All values in repeating quantifiers must be less than 65536.

       There is no limit to the number of parenthesized subpatterns, but there
       can be no more than 65535 capturing subpatterns.

       There is a limit to the number of forward references to subsequent sub-
       patterns of around 200,000.  Repeated  forward  references  with  fixed
       upper  limits,  for example, (?2){0,100} when subpattern number 2 is to
       the right, are included in the count. There is no limit to  the  number
       of backward references.

       The maximum length of name for a named subpattern is 32 characters, and
       the maximum number of named subpatterns is 10000.

       The maximum length of a  name  in  a  (*MARK),  (*PRUNE),  (*SKIP),  or
       (*THEN)  verb  is  255  for  the 8-bit library and 65535 for the 16-bit
       library.

       The maximum length of a subject string is the largest  positive  number
       that  an integer variable can hold. However, when using the traditional
       matching function, PCRE uses recursion to handle subpatterns and indef-
       inite  repetition.  This means that the available stack space may limit
       the size of a subject string that can be processed by certain patterns.
       For a discussion of stack issues, see the pcrestack documentation.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 04 May 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRESTACK(3)                                                      PCRESTACK(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE DISCUSSION OF STACK USAGE

       When  you  call  pcre[16]_exec(),  it makes use of an internal function
       called match(). This calls itself recursively at branch points  in  the
       pattern,  in  order  to  remember the state of the match so that it can
       back up and try a different alternative if  the  first  one  fails.  As
       matching proceeds deeper and deeper into the tree of possibilities, the
       recursion depth increases. The match() function is also called in other
       circumstances,  for  example,  whenever  a parenthesized sub-pattern is
       entered, and in certain cases of repetition.

       Not all calls of match() increase the recursion depth; for an item such
       as  a* it may be called several times at the same level, after matching
       different numbers of a's. Furthermore, in a number of cases  where  the
       result  of  the  recursive call would immediately be passed back as the
       result of the current call (a "tail recursion"), the function  is  just
       restarted instead.

       The  above  comments  apply  when  pcre[16]_exec() is run in its normal
       interpretive  manner.   If   the   pattern   was   studied   with   the
       PCRE_STUDY_JIT_COMPILE  option, and just-in-time compiling was success-
       ful, and the options passed to pcre[16]_exec() were  not  incompatible,
       the  matching process uses the JIT-compiled code instead of the match()
       function. In this case, the memory requirements  are  handled  entirely
       differently. See the pcrejit documentation for details.

       The pcre[16]_dfa_exec() function operates in an entirely different way,
       and uses recursion only when there is a regular expression recursion or
       subroutine  call in the pattern. This includes the processing of asser-
       tion and "once-only" subpatterns, which  are  handled  like  subroutine
       calls.  Normally,  these are never very deep, and the limit on the com-
       plexity of pcre[16]_dfa_exec() is controlled by the amount of workspace
       it  is  given.   However, it is possible to write patterns with runaway
       infinite recursions; such patterns will  cause  pcre[16]_dfa_exec()  to
       run out of stack. At present, there is no protection against this.

       The  comments that follow do NOT apply to pcre[16]_dfa_exec(); they are
       relevant only for pcre[16]_exec() without the JIT optimization.

   Reducing pcre[16]_exec()'s stack usage

       Each time that match() is actually called recursively, it  uses  memory
       from  the  process  stack.  For certain kinds of pattern and data, very
       large amounts of stack may be needed, despite the recognition of  "tail
       recursion".   You  can often reduce the amount of recursion, and there-
       fore the amount of stack used, by modifying the pattern that  is  being
       matched. Consider, for example, this pattern:

         ([^<]|<(?!inet))+

       It  matches  from wherever it starts until it encounters "<inet" or the
       end of the data, and is the kind of pattern that  might  be  used  when
       processing an XML file. Each iteration of the outer parentheses matches
       either one character that is not "<" or a "<" that is not  followed  by
       "inet".  However,  each  time  a  parenthesis is processed, a recursion
       occurs, so this formulation uses a stack frame for each matched charac-
       ter.  For  a long string, a lot of stack is required. Consider now this
       rewritten pattern, which matches exactly the same strings:

         ([^<]++|<(?!inet))+

       This uses very much less stack, because runs of characters that do  not
       contain  "<" are "swallowed" in one item inside the parentheses. Recur-
       sion happens only when a "<" character that is not followed  by  "inet"
       is  encountered  (and  we assume this is relatively rare). A possessive
       quantifier is used to stop any backtracking into the  runs  of  non-"<"
       characters, but that is not related to stack usage.

       This  example shows that one way of avoiding stack problems when match-
       ing long subject strings is to write repeated parenthesized subpatterns
       to match more than one character whenever possible.

   Compiling PCRE to use heap instead of stack for pcre[16]_exec()

       In  environments  where  stack memory is constrained, you might want to
       compile PCRE to use heap memory instead of stack for remembering  back-
       up points when pcre[16]_exec() is running. This makes it run a lot more
       slowly, however.  Details of how to do this are given in the  pcrebuild
       documentation. When built in this way, instead of using the stack, PCRE
       obtains and frees memory by calling the functions that are  pointed  to
       by  the  pcre[16]_stack_malloc  and  pcre[16]_stack_free  variables. By
       default, these point to malloc() and free(), but you  can  replace  the
       pointers to cause PCRE to use your own functions. Since the block sizes
       are always the same, and are always freed in reverse order, it  may  be
       possible  to  implement  customized memory handlers that are more effi-
       cient than the standard functions.

   Limiting pcre[16]_exec()'s stack usage

       You can set limits on the number of times that match() is called,  both
       in  total  and  recursively.  If  a  limit is exceeded, pcre[16]_exec()
       returns an error code. Setting suitable limits should prevent  it  from
       running  out of stack. The default values of the limits are very large,
       and unlikely ever to operate. They can be changed when PCRE  is  built,
       and they can also be set when pcre[16]_exec() is called. For details of
       these interfaces, see the pcrebuild documentation and  the  section  on
       extra data for pcre[16]_exec() in the pcreapi documentation.

       As a very rough rule of thumb, you should reckon on about 500 bytes per
       recursion. Thus, if you want to limit your  stack  usage  to  8Mb,  you
       should  set  the  limit at 16000 recursions. A 64Mb stack, on the other
       hand, can support around 128000 recursions.

       In Unix-like environments, the pcretest test program has a command line
       option (-S) that can be used to increase the size of its stack. As long
       as the stack is large enough, another option (-M) can be used  to  find
       the  smallest  limits  that allow a particular pattern to match a given
       subject string. This is done by calling pcre[16]_exec() repeatedly with
       different limits.

   Obtaining an estimate of stack usage

       The  actual  amount  of  stack used per recursion can vary quite a lot,
       depending on the compiler that was used to build PCRE and the optimiza-
       tion or debugging options that were set for it. The rule of thumb value
       of 500 bytes mentioned above may be larger  or  smaller  than  what  is
       actually needed. A better approximation can be obtained by running this
       command:

         pcretest -m -C

       The -C option causes pcretest to output information about  the  options
       with which PCRE was compiled. When -m is also given (before -C), infor-
       mation about stack use is given in a line like this:

         Match recursion uses stack: approximate frame size = 640 bytes

       The value is approximate because some recursions need a bit more (up to
       perhaps 16 more bytes).

       If  the  above  command  is given when PCRE is compiled to use the heap
       instead of the stack for recursion, the value that  is  output  is  the
       size of each block that is obtained from the heap.

   Changing stack size in Unix-like systems

       In  Unix-like environments, there is not often a problem with the stack
       unless very long strings are involved,  though  the  default  limit  on
       stack  size  varies  from system to system. Values from 8Mb to 64Mb are
       common. You can find your default limit by running the command:

         ulimit -s

       Unfortunately, the effect of running out of  stack  is  often  SIGSEGV,
       though  sometimes  a more explicit error message is given. You can nor-
       mally increase the limit on stack size by code such as this:

         struct rlimit rlim;
         getrlimit(RLIMIT_STACK, &rlim);
         rlim.rlim_cur = 100*1024*1024;
         setrlimit(RLIMIT_STACK, &rlim);

       This reads the current limits (soft and hard) using  getrlimit(),  then
       attempts  to  increase  the  soft limit to 100Mb using setrlimit(). You
       must do this before calling pcre[16]_exec().

   Changing stack size in Mac OS X

       Using setrlimit(), as described above, should also work on Mac OS X. It
       is also possible to set a stack size when linking a program. There is a
       discussion  about  stack  sizes  in  Mac  OS  X  at  this   web   site:
       http://developer.apple.com/qa/qa2005/qa1419.html.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 21 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


