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       PCRE - Perl-compatible regular expressions (original API)


       This  document  relates  to  PCRE  releases  that use the original API, with library names
       libpcre, libpcre16, and libpcre32. January 2015 saw the first release of a new API,  known
       as   PCRE2,  with  release  numbers  starting  at  10.00  and  library  names  libpcre2-8,
       libpcre2-16, and libpcre2-32. The  old  libraries  (now  called  PCRE1)  are  still  being
       maintained  for  bug fixes, but there will be no new development. New projects are advised
       to use the new PCRE2 libraries.


       The PCRE library is a set of functions that implement regular expression 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  syntax,  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

       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.

       Starting  with  release  8.32 it is possible to compile a third separate PCRE library that
       supports 32-bit character strings (including UTF-32 strings). The build process allows any
       combination  of  the 8-, 16- and 32-bit libraries. The work to make this possible was done
       by Christian Persch.

       The three libraries contain identical sets of functions, except  that  the  names  in  the
       16-bit  library  start  with pcre16_ instead of pcre_, and the names in the 32-bit library
       start  with  pcre32_  instead  of  pcre_.  To  avoid  over-complication  and  reduce   the
       documentation  maintenance  load,  most  of the documentation describes the 8-bit library,
       with the differences for the 16-bit and  32-bit  libraries  described  separately  in  the
       pcre16 and pcre32 pages. References to functions or structures of the form pcre[16|32]_xxx
       should be read as meaning "pcre_xxx when using the 8-bit library,  pcre16_xxx  when  using
       the 16-bit library, or pcre32_xxx when using the 32-bit library".

       The  current  implementation  of  PCRE corresponds approximately with Perl 5.12, including
       support for UTF-8/16/32 encoded strings and Unicode general category properties.  However,
       UTF-8/16/32  and  Unicode support has to be explicitly enabled; it is not the default. The
       Unicode tables correspond to Unicode release 6.3.0.

       In addition to  the  Perl-compatible  matching  function,  PCRE  contains  an  alternative
       function  that  matches  the  same  compiled  patterns  in  a  different  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 contributions can be  found
       in the Contrib directory at the primary FTP site, which is:

       Details  of  exactly  which  Perl regular expression features are and are not supported by
       PCRE are given in separate documents. See the pcrepattern 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  themselves  are  described in the pcrebuild page. Documentation
       about building PCRE for various operating systems can be found  in  the  README  and  NON-
       AUTOTOOLS_BUILD 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_" or
       "_pcre32_", 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.


       If you are using PCRE in a non-UTF application that  permits  users  to  supply  arbitrary
       patterns  for  compilation,  you should be aware of a feature that allows users to turn on
       UTF support from within a pattern, provided that PCRE was  built  with  UTF  support.  For
       example,  an  8-bit  pattern  that  begins with "(*UTF8)" or "(*UTF)" turns on UTF-8 mode,
       which interprets  patterns  and  subjects  as  strings  of  UTF-8  characters  instead  of
       individual  8-bit  characters.  This causes both the pattern and any data against which it
       is matched to be checked for UTF-8 validity. If the data string is very long, such a check
       might use sufficiently many resources as to cause your application to lose performance.

       One  way  of  guarding  against this possibility is to use the pcre_fullinfo() function to
       check the compiled pattern's options for UTF.  Alternatively, from release 8.33,  you  can
       set  the  PCRE_NEVER_UTF  option  at  compile time. This causes an compile time error if a
       pattern contains a UTF-setting sequence.

       If your application is one that supports UTF, be aware that  validity  checking  can  take
       time.   If  the  same  data  string  is  to  be  matched  many  times,  you  can  use  the
       PCRE_NO_UTF[8|16|32]_CHECK option for the second and subsequent matches to save  redundant

       Another  way  that  performance  can  be hit is by running a pattern that has a very large
       search tree against a string that will never match. Nested unlimited repeats in a  pattern
       are   a   common   example.   PCRE   provides   some  protection  against  this:  see  the
       PCRE_EXTRA_MATCH_LIMIT feature in the pcreapi page.


       The user documentation for PCRE comprises a number of different  sections.  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,  the  descriptions  of  the
       pcregrep  and  pcretest  programs  are  in  files  called  pcregrep.txt  and pcretest.txt,
       respectively. The remaining sections, except for the pcredemo section (which is a  program
       listing),  are  concatenated  in  pcre.txt,  for  ease  of  searching. The sections are as

         pcre              this document
         pcre-config       show PCRE installation configuration information
         pcre16            details of the 16-bit library
         pcre32            details of the 32-bit library
         pcreapi           details of PCRE's native C API
         pcrebuild         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-8/16/32 support

       In the "man" and HTML formats, there is also a short page for  each  C  library  function,
       listing its arguments and results.


       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


       Last updated: 10 February 2015
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