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libraries/spcre/libpcre/pcre/doc/html/pcre.html
author Tom Sutcliffe <thomas.sutcliffe@accenture.com>
Sat, 31 Jul 2010 19:07:57 +0100
changeset 23 092bcc217d9d
parent 0 7f656887cf89
permissions -rw-r--r--
Tidied iocli exports, build macro tweaks. Removed 4 overloads of CCommandBase::RunCommand[L] that are no longer used at all, and changed one more to not be exported as it's only used internally to iocli.dll. fixed builds on platforms that don't support btrace or any form of tracing.
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<html>
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<head>
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<title>pcre specification</title>
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</head>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcre man page</h1>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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<p>
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This page is part of the PCRE HTML documentation. It was generated automatically
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from the original man page. If there is any nonsense in it, please consult the
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man page, in case the conversion went wrong.
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<br>
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<ul>
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<li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
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<li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
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<li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
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<li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
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<li><a name="TOC5" href="#SEC5">AUTHOR</a>
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<li><a name="TOC6" href="#SEC6">REVISION</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
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<P>
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The PCRE library is a set of functions that implement regular expression
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pattern matching using the same syntax and semantics as Perl, with just a few
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differences. Certain features that appeared in Python and PCRE before they
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appeared in Perl are also available using the Python syntax. There is also some
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support for certain .NET and Oniguruma syntax items, and there is an option for
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requesting some minor changes that give better JavaScript compatibility.
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</P>
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<P>
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The current implementation of PCRE (release 7.x) corresponds approximately with
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Perl 5.10, including support for UTF-8 encoded strings and Unicode general
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category properties. However, UTF-8 and Unicode support has to be explicitly
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enabled; it is not the default. The Unicode tables correspond to Unicode
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release 5.0.0.
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</P>
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<P>
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In addition to the Perl-compatible matching function, PCRE contains an
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alternative matching function that matches the same compiled patterns in a
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different way. In certain circumstances, the alternative function has some
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advantages. For a discussion of the two matching algorithms, see the
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<a href="pcrematching.html"><b>pcrematching</b></a>
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page.
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</P>
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<P>
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PCRE is written in C and released as a C library. A number of people have
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written wrappers and interfaces of various kinds. In particular, Google Inc.
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have provided a comprehensive C++ wrapper. This is now included as part of the
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PCRE distribution. The
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<a href="pcrecpp.html"><b>pcrecpp</b></a>
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page has details of this interface. Other people's contributions can be found
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in the <i>Contrib</i> directory at the primary FTP site, which is:
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<a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
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</P>
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<P>
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Details of exactly which Perl regular expression features are and are not
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supported by PCRE are given in separate documents. See the
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<a href="pcrepattern.html"><b>pcrepattern</b></a>
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and
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<a href="pcrecompat.html"><b>pcrecompat</b></a>
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pages. There is a syntax summary in the
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<a href="pcresyntax.html"><b>pcresyntax</b></a>
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page.
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</P>
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<P>
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Some features of PCRE can be included, excluded, or changed when the library is
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built. The
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<a href="pcre_config.html"><b>pcre_config()</b></a>
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function makes it possible for a client to discover which features are
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available. The features themselves are described in the
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<a href="pcrebuild.html"><b>pcrebuild</b></a>
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page. Documentation about building PCRE for various operating systems can be
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found in the <b>README</b> file in the source distribution.
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</P>
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<P>
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The library contains a number of undocumented internal functions and data
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tables that are used by more than one of the exported external functions, but
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which are not intended for use by external callers. Their names all begin with
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"_pcre_", which hopefully will not provoke any name clashes. In some
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environments, it is possible to control which external symbols are exported
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when a shared library is built, and in these cases the undocumented symbols are
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not exported.
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</P>
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<br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
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<P>
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The user documentation for PCRE comprises a number of different sections. In
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the "man" format, each of these is a separate "man page". In the HTML format,
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each is a separate page, linked from the index page. In the plain text format,
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all the sections are concatenated, for ease of searching. The sections are as
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follows:
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<pre>
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  pcre              this document
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  pcre-config       show PCRE installation configuration information
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  pcreapi           details of PCRE's native C API
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  pcrebuild         options for building PCRE
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  pcrecallout       details of the callout feature
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  pcrecompat        discussion of Perl compatibility
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  pcrecpp           details of the C++ wrapper
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  pcregrep          description of the <b>pcregrep</b> command
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  pcrematching      discussion of the two matching algorithms
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  pcrepartial       details of the partial matching facility
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  pcrepattern       syntax and semantics of supported regular expressions
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  pcresyntax        quick syntax reference
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  pcreperform       discussion of performance issues
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  pcreposix         the POSIX-compatible C API
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  pcreprecompile    details of saving and re-using precompiled patterns
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  pcresample        discussion of the sample program
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  pcrestack         discussion of stack usage
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  pcretest          description of the <b>pcretest</b> testing command
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</pre>
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In addition, in the "man" and HTML formats, there is a short page for each
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C library function, listing its arguments and results.
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</P>
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<br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
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<P>
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There are some size limitations in PCRE but it is hoped that they will never in
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practice be relevant.
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</P>
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<P>
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The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
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compiled with the default internal linkage size of 2. If you want to process
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regular expressions that are truly enormous, you can compile PCRE with an
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internal linkage size of 3 or 4 (see the <b>README</b> file in the source
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distribution and the
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<a href="pcrebuild.html"><b>pcrebuild</b></a>
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documentation for details). In these cases the limit is substantially larger.
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However, the speed of execution is slower.
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</P>
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<P>
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All values in repeating quantifiers must be less than 65536.
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</P>
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<P>
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There is no limit to the number of parenthesized subpatterns, but there can be
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no more than 65535 capturing subpatterns.
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</P>
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<P>
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The maximum length of name for a named subpattern is 32 characters, and the
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maximum number of named subpatterns is 10000.
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</P>
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<P>
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The maximum length of a subject string is the largest positive number that an
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integer variable can hold. However, when using the traditional matching
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function, PCRE uses recursion to handle subpatterns and indefinite repetition.
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This means that the available stack space may limit the size of a subject
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string that can be processed by certain patterns. For a discussion of stack
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issues, see the
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<a href="pcrestack.html"><b>pcrestack</b></a>
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documentation.
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<a name="utf8support"></a></P>
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<br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
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<P>
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From release 3.3, PCRE has had some support for character strings encoded in
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the UTF-8 format. For release 4.0 this was greatly extended to cover most
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common requirements, and in release 5.0 additional support for Unicode general
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category properties was added.
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</P>
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<P>
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In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
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the code, and, in addition, you must call
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<a href="pcre_compile.html"><b>pcre_compile()</b></a>
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with the PCRE_UTF8 option flag. When you do this, both the pattern and any
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subject strings that are matched against it are treated as UTF-8 strings
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instead of just strings of bytes.
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</P>
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<P>
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If you compile PCRE with UTF-8 support, but do not use it at run time, the
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library will be a bit bigger, but the additional run time overhead is limited
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to testing the PCRE_UTF8 flag occasionally, so should not be very big.
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</P>
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<P>
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If PCRE is built with Unicode character property support (which implies UTF-8
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support), the escape sequences \p{..}, \P{..}, and \X are supported.
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The available properties that can be tested are limited to the general
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category properties such as Lu for an upper case letter or Nd for a decimal
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number, the Unicode script names such as Arabic or Han, and the derived
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properties Any and L&. A full list is given in the
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<a href="pcrepattern.html"><b>pcrepattern</b></a>
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documentation. Only the short names for properties are supported. For example,
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\p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
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Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
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compatibility with Perl 5.6. PCRE does not support this.
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<a name="utf8strings"></a></P>
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<br><b>
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Validity of UTF-8 strings
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</b><br>
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<P>
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When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
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are (by default) checked for validity on entry to the relevant functions. From
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release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
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themselves derived from the Unicode specification. Earlier releases of PCRE
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followed the rules of RFC 2279, which allows the full range of 31-bit values (0
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to 0x7FFFFFFF). The current check allows only values in the range U+0 to
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U+10FFFF, excluding U+D800 to U+DFFF.
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</P>
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<P>
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The excluded code points are the "Low Surrogate Area" of Unicode, of which the
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Unicode Standard says this: "The Low Surrogate Area does not contain any
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character assignments, consequently no character code charts or namelists are
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provided for this area. Surrogates are reserved for use with UTF-16 and then
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must be used in pairs." The code points that are encoded by UTF-16 pairs are
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available as independent code points in the UTF-8 encoding. (In other words,
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the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
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UTF-8.)
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</P>
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<P>
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If an invalid UTF-8 string is passed to PCRE, an error return
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(PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
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your strings are valid, and therefore want to skip these checks in order to
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improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
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at run time, PCRE assumes that the pattern or subject it is given
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(respectively) contains only valid UTF-8 codes. In this case, it does not
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diagnose an invalid UTF-8 string.
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</P>
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<P>
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If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
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happens depends on why the string is invalid. If the string conforms to the
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"old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
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in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
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test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
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rules of RFC 2279. However, if the string does not even conform to RFC 2279,
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the result is undefined. Your program may crash.
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</P>
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<P>
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If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
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encoded in a UTF-8-like manner as per the old RFC, you can set
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PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
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situation, you will have to apply your own validity check.
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</P>
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<br><b>
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General comments about UTF-8 mode
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</b><br>
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<P>
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1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
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UTF-8 character if the value is greater than 127.
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</P>
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<P>
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2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
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characters for values greater than \177.
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</P>
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<P>
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3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
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bytes, for example: \x{100}{3}.
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</P>
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<P>
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4. The dot metacharacter matches one UTF-8 character instead of a single byte.
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5. The escape sequence \C can be used to match a single byte in UTF-8 mode,
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but its use can lead to some strange effects. This facility is not available in
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the alternative matching function, <b>pcre_dfa_exec()</b>.
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6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
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test characters of any code value, but the characters that PCRE recognizes as
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digits, spaces, or word characters remain the same set as before, all with
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values less than 256. This remains true even when PCRE includes Unicode
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property support, because to do otherwise would slow down PCRE in many common
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cases. If you really want to test for a wider sense of, say, "digit", you
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must use Unicode property tests such as \p{Nd}.
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7. Similarly, characters that match the POSIX named character classes are all
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low-valued characters.
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8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
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(\h, \H, \v, and \V) do match all the appropriate Unicode characters.
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<P>
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9. Case-insensitive matching applies only to characters whose values are less
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than 128, unless PCRE is built with Unicode property support. Even when Unicode
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property support is available, PCRE still uses its own character tables when
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checking the case of low-valued characters, so as not to degrade performance.
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The Unicode property information is used only for characters with higher
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values. Even when Unicode property support is available, PCRE supports
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case-insensitive matching only when there is a one-to-one mapping between a
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letter's cases. There are a small number of many-to-one mappings in Unicode;
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these are not supported by PCRE.
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<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
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<P>
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Philip Hazel
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<br>
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University Computing Service
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<br>
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Cambridge CB2 3QH, England.
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<br>
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</P>
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<P>
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Putting an actual email address here seems to have been a spam magnet, so I've
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taken it away. If you want to email me, use my two initials, followed by the
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two digits 10, at the domain cam.ac.uk.
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</P>
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<br><a name="SEC6" href="#TOC1">REVISION</a><br>
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<P>
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Last updated: 12 April 2008
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<br>
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Copyright &copy; 1997-2008 University of Cambridge.
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<br>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
FCL/sf/os/fshell