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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.

       Starting with release 8.32 it is possible to compile a  third  separate
       PCRE library, which supports 32-bit character strings (including UTF-32
       strings). The build process allows any set 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  mainte-
       nance load, most of the documentation describes the 8-bit library, with
       the differences for the 16-bit and  32-bit  libraries  described  sepa-
       rately  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 6.2.0.

       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:

       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.


SECURITY CONSIDERATIONS


       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 valid-
       ity.  If  the  data  string is very long, such a check might use suffi-
       ciently many resources as to cause your  application  to  lose  perfor-
       mance.

       The  best  way  of  guarding  against  this  possibility  is to use the
       pcre_fullinfo() function to check the compiled  pattern's  options  for
       UTF.

       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 checks.

       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  pro-
       vides some protection against this: see the PCRE_EXTRA_MATCH_LIMIT fea-
       ture in the pcreapi page.


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
         pcre32            details of the 32-bit library
         pcre-config       show PCRE installation configuration information
         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  addition,  in the "man" and HTML formats, there is a short page for
       each 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: 11 November 2012
       Copyright (c) 1997-2012 University of Cambridge.


PCRE 8.32 11 November 2012 PCRE(3)



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