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 PCRE(3)                          PCRE 8.37                          PCRE(3)
                              10 February 2015

      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 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

      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

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 PCRE(3)                          PCRE 8.37                          PCRE(3)
                              10 February 2015

      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

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                              10 February 2015


      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 a 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.


      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
        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

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 PCRE(3)                          PCRE 8.37                          PCRE(3)
                              10 February 2015

        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 domain


      Last updated: 10 February 2015
      Copyright (c) 1997-2015 University of Cambridge.

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