NAME

       PCRE - Perl-compatible regular expressions (original API)

PLEASE TAKE NOTE

       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.

INTRODUCTION

       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 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 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:

       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 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.

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 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 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 provides
       some protection against this: see the PCRE_EXTRA_MATCH_LIMIT feature in the pcreapi page.

USER DOCUMENTATION

       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
       follows:

         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.

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 February 2015
       Copyright (c) 1997-2015 University of Cambridge.

PCRE 8.37                                       10 February 2015                                         PCRE(3)