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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 869 by ph10, Sat Jan 14 11:16:23 2012 UTC
# Line 15  man page, in case the conversion went wr Line 15  man page, in case the conversion went wr
15  <ul>  <ul>
16  <li><a name="TOC1" href="#SEC1">INTRODUCTION</a>  <li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
17  <li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>  <li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
18  <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>  <li><a name="TOC3" href="#SEC3">AUTHOR</a>
19  <li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>  <li><a name="TOC4" href="#SEC4">REVISION</a>
 <li><a name="TOC5" href="#SEC5">AUTHOR</a>  
20  </ul>  </ul>
21  <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>  <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
22  <P>  <P>
23  The PCRE library is a set of functions that implement regular expression  The PCRE library is a set of functions that implement regular expression
24  pattern matching using the same syntax and semantics as Perl, with just a few  pattern matching using the same syntax and semantics as Perl, with just a few
25  differences. The current implementation of PCRE (release 6.x) corresponds  differences. Some features that appeared in Python and PCRE before they
26  approximately with Perl 5.8, including support for UTF-8 encoded strings and  appeared in Perl are also available using the Python syntax, there is some
27  Unicode general category properties. However, this support has to be explicitly  support for one or two .NET and Oniguruma syntax items, and there is an option
28  enabled; it is not the default.  for requesting some minor changes that give better JavaScript compatibility.
29  </P>  </P>
30  <P>  <P>
31  In addition to the Perl-compatible matching function, PCRE also contains an  Starting with release 8.30, it is possible to compile two separate PCRE
32  alternative matching function that matches the same compiled patterns in a  libraries: the original, which supports 8-bit character strings (including
33  different way. In certain circumstances, the alternative function has some  UTF-8 strings), and a second library that supports 16-bit character strings
34  advantages. For a discussion of the two matching algorithms, see the  (including UTF-16 strings). The build process allows either one or both to be
35    built. The majority of the work to make this possible was done by Zoltan
36    Herczeg.
37    </P>
38    <P>
39    The two libraries contain identical sets of functions, except that the names in
40    the 16-bit library start with <b>pcre16_</b> instead of <b>pcre_</b>. To avoid
41    over-complication and reduce the documentation maintenance load, most of the
42    documentation describes the 8-bit library, with the differences for the 16-bit
43    library described separately in the
44    <a href="pcre16.html"><b>pcre16</b></a>
45    page. References to functions or structures of the form <i>pcre[16]_xxx</i>
46    should be read as meaning "<i>pcre_xxx</i> when using the 8-bit library and
47    <i>pcre16_xxx</i> when using the 16-bit library".
48    </P>
49    <P>
50    The current implementation of PCRE corresponds approximately with Perl 5.12,
51    including support for UTF-8/16 encoded strings and Unicode general category
52    properties. However, UTF-8/16 and Unicode support has to be explicitly enabled;
53    it is not the default. The Unicode tables correspond to Unicode release 6.0.0.
54    </P>
55    <P>
56    In addition to the Perl-compatible matching function, PCRE contains an
57    alternative function that matches the same compiled patterns in a different
58    way. In certain circumstances, the alternative function has some advantages.
59    For a discussion of the two matching algorithms, see the
60  <a href="pcrematching.html"><b>pcrematching</b></a>  <a href="pcrematching.html"><b>pcrematching</b></a>
61  page.  page.
62  </P>  </P>
63  <P>  <P>
64  PCRE is written in C and released as a C library. A number of people have  PCRE is written in C and released as a C library. A number of people have
65  written wrappers and interfaces of various kinds. In particular, Google Inc.  written wrappers and interfaces of various kinds. In particular, Google Inc.
66  have provided a comprehensive C++ wrapper. This is now included as part of the  have provided a comprehensive C++ wrapper for the 8-bit library. This is now
67  PCRE distribution. The  included as part of the PCRE distribution. The
68  <a href="pcrecpp.html"><b>pcrecpp</b></a>  <a href="pcrecpp.html"><b>pcrecpp</b></a>
69  page has details of this interface. Other people's contributions can be found  page has details of this interface. Other people's contributions can be found
70  in the <i>Contrib</i> directory at the primary FTP site, which is:  in the <i>Contrib</i> directory at the primary FTP site, which is:
# Line 52  supported by PCRE are given in separate Line 76  supported by PCRE are given in separate
76  <a href="pcrepattern.html"><b>pcrepattern</b></a>  <a href="pcrepattern.html"><b>pcrepattern</b></a>
77  and  and
78  <a href="pcrecompat.html"><b>pcrecompat</b></a>  <a href="pcrecompat.html"><b>pcrecompat</b></a>
79  pages.  pages. There is a syntax summary in the
80    <a href="pcresyntax.html"><b>pcresyntax</b></a>
81    page.
82  </P>  </P>
83  <P>  <P>
84  Some features of PCRE can be included, excluded, or changed when the library is  Some features of PCRE can be included, excluded, or changed when the library is
# Line 62  function makes it possible for a client Line 88  function makes it possible for a client
88  available. The features themselves are described in the  available. The features themselves are described in the
89  <a href="pcrebuild.html"><b>pcrebuild</b></a>  <a href="pcrebuild.html"><b>pcrebuild</b></a>
90  page. Documentation about building PCRE for various operating systems can be  page. Documentation about building PCRE for various operating systems can be
91  found in the <b>README</b> file in the source distribution.  found in the <b>README</b> and <b>NON-UNIX-USE</b> files in the source
92    distribution.
93  </P>  </P>
94  <P>  <P>
95  The library contains a number of undocumented internal functions and data  The libraries contains a number of undocumented internal functions and data
96  tables that are used by more than one of the exported external functions, but  tables that are used by more than one of the exported external functions, but
97  which are not intended for use by external callers. Their names all begin with  which are not intended for use by external callers. Their names all begin with
98  "_pcre_", which hopefully will not provoke any name clashes. In some  "_pcre_" or "_pcre16_", which hopefully will not provoke any name clashes. In
99  environments, it is possible to control which external symbols are exported  some environments, it is possible to control which external symbols are
100  when a shared library is built, and in these cases the undocumented symbols are  exported when a shared library is built, and in these cases the undocumented
101  not exported.  symbols are not exported.
102  </P>  </P>
103  <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>  <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
104  <P>  <P>
105  The user documentation for PCRE comprises a number of different sections. In  The user documentation for PCRE comprises a number of different sections. In
106  the "man" format, each of these is a separate "man page". In the HTML format,  the "man" format, each of these is a separate "man page". In the HTML format,
107  each is a separate page, linked from the index page. In the plain text format,  each is a separate page, linked from the index page. In the plain text format,
108  all the sections are concatenated, for ease of searching. The sections are as  all the sections, except the <b>pcredemo</b> section, are concatenated, for ease
109  follows:  of searching. The sections are as follows:
110  <pre>  <pre>
111    pcre              this document    pcre              this document
112      pcre16            details of the 16-bit library
113      pcre-config       show PCRE installation configuration information
114    pcreapi           details of PCRE's native C API    pcreapi           details of PCRE's native C API
115    pcrebuild         options for building PCRE    pcrebuild         options for building PCRE
116    pcrecallout       details of the callout feature    pcrecallout       details of the callout feature
117    pcrecompat        discussion of Perl compatibility    pcrecompat        discussion of Perl compatibility
118    pcrecpp           details of the C++ wrapper    pcrecpp           details of the C++ wrapper for the 8-bit library
119    pcregrep          description of the <b>pcregrep</b> command    pcredemo          a demonstration C program that uses PCRE
120      pcregrep          description of the <b>pcregrep</b> command (8-bit only)
121      pcrejit           discussion of the just-in-time optimization support
122      pcrelimits        details of size and other limits
123    pcrematching      discussion of the two matching algorithms    pcrematching      discussion of the two matching algorithms
124    pcrepartial       details of the partial matching facility    pcrepartial       details of the partial matching facility
125    pcrepattern       syntax and semantics of supported regular expressions    pcrepattern       syntax and semantics of supported regular expressions
126    pcreperform       discussion of performance issues    pcreperform       discussion of performance issues
127    pcreposix         the POSIX-compatible C API    pcreposix         the POSIX-compatible C API for the 8-bit library
128    pcreprecompile    details of saving and re-using precompiled patterns    pcreprecompile    details of saving and re-using precompiled patterns
129    pcresample        discussion of the sample program    pcresample        discussion of the pcredemo program
130    pcrestack         discussion of stack usage    pcrestack         discussion of stack usage
131      pcresyntax        quick syntax reference
132    pcretest          description of the <b>pcretest</b> testing command    pcretest          description of the <b>pcretest</b> testing command
133      pcreunicode       discussion of Unicode and UTF-8/16 support
134  </pre>  </pre>
135  In addition, in the "man" and HTML formats, there is a short page for each  In addition, in the "man" and HTML formats, there is a short page for each
136  C library function, listing its arguments and results.  8-bit C library function, listing its arguments and results.
 </P>  
 <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>  
 <P>  
 There are some size limitations in PCRE but it is hoped that they will never in  
 practice be relevant.  
 </P>  
 <P>  
 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is  
 compiled with the default internal linkage size of 2. If you want to process  
 regular expressions that are truly enormous, you can compile PCRE with an  
 internal linkage size of 3 or 4 (see the <b>README</b> file in the source  
 distribution and the  
 <a href="pcrebuild.html"><b>pcrebuild</b></a>  
 documentation for details). In these cases the limit is substantially larger.  
 However, the speed of execution will be slower.  
 </P>  
 <P>  
 All values in repeating quantifiers must be less than 65536. The maximum  
 compiled length of subpattern with an explicit repeat count is 30000 bytes. The  
 maximum number of capturing subpatterns is 65535.  
 </P>  
 <P>  
 There is no limit to the number of non-capturing subpatterns, but the maximum  
 depth of nesting of all kinds of parenthesized subpattern, including capturing  
 subpatterns, assertions, and other types of subpattern, is 200.  
 </P>  
 <P>  
 The maximum length of name for a named subpattern is 32, and the maximum number  
 of named subpatterns is 10000.  
 </P>  
 <P>  
 The maximum length of a subject string is the largest positive number that an  
 integer variable can hold. However, when using the traditional matching  
 function, PCRE uses recursion to handle subpatterns and indefinite repetition.  
 This means that the available stack space may limit the size of a subject  
 string that can be processed by certain patterns. For a discussion of stack  
 issues, see the  
 <a href="pcrestack.html"><b>pcrestack</b></a>  
 documentation.  
 <a name="utf8support"></a></P>  
 <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>  
 <P>  
 From release 3.3, PCRE has had some support for character strings encoded in  
 the UTF-8 format. For release 4.0 this was greatly extended to cover most  
 common requirements, and in release 5.0 additional support for Unicode general  
 category properties was added.  
 </P>  
 <P>  
 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in  
 the code, and, in addition, you must call  
 <a href="pcre_compile.html"><b>pcre_compile()</b></a>  
 with the PCRE_UTF8 option flag. When you do this, both the pattern and any  
 subject strings that are matched against it are treated as UTF-8 strings  
 instead of just strings of bytes.  
 </P>  
 <P>  
 If you compile PCRE with UTF-8 support, but do not use it at run time, the  
 library will be a bit bigger, but the additional run time overhead is limited  
 to testing the PCRE_UTF8 flag in several places, so should not be very large.  
 </P>  
 <P>  
 If PCRE is built with Unicode character property support (which implies UTF-8  
 support), the escape sequences \p{..}, \P{..}, and \X are supported.  
 The available properties that can be tested are limited to the general  
 category properties such as Lu for an upper case letter or Nd for a decimal  
 number, the Unicode script names such as Arabic or Han, and the derived  
 properties Any and L&. A full list is given in the  
 <a href="pcrepattern.html"><b>pcrepattern</b></a>  
 documentation. Only the short names for properties are supported. For example,  
 \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.  
 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for  
 compatibility with Perl 5.6. PCRE does not support this.  
137  </P>  </P>
138  <P>  <br><a name="SEC3" href="#TOC1">AUTHOR</a><br>
 The following comments apply when PCRE is running in UTF-8 mode:  
 </P>  
 <P>  
 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects  
 are checked for validity on entry to the relevant functions. If an invalid  
 UTF-8 string is passed, an error return is given. In some situations, you may  
 already know that your strings are valid, and therefore want to skip these  
 checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag  
 at compile time or at run time, PCRE assumes that the pattern or subject it  
 is given (respectively) contains only valid UTF-8 codes. In this case, it does  
 not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to  
 PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program  
 may crash.  
 </P>  
 <P>  
 2. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte  
 UTF-8 character if the value is greater than 127.  
 </P>  
 <P>  
 3. Octal numbers up to \777 are recognized, and match two-byte UTF-8  
 characters for values greater than \177.  
 </P>  
 <P>  
 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual  
 bytes, for example: \x{100}{3}.  
 </P>  
 <P>  
 5. The dot metacharacter matches one UTF-8 character instead of a single byte.  
 </P>  
 <P>  
 6. The escape sequence \C can be used to match a single byte in UTF-8 mode,  
 but its use can lead to some strange effects. This facility is not available in  
 the alternative matching function, <b>pcre_dfa_exec()</b>.  
 </P>  
 <P>  
 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly  
 test characters of any code value, but the characters that PCRE recognizes as  
 digits, spaces, or word characters remain the same set as before, all with  
 values less than 256. This remains true even when PCRE includes Unicode  
 property support, because to do otherwise would slow down PCRE in many common  
 cases. If you really want to test for a wider sense of, say, "digit", you  
 must use Unicode property tests such as \p{Nd}.  
 </P>  
 <P>  
 8. Similarly, characters that match the POSIX named character classes are all  
 low-valued characters.  
 </P>  
 <P>  
 9. Case-insensitive matching applies only to characters whose values are less  
 than 128, unless PCRE is built with Unicode property support. Even when Unicode  
 property support is available, PCRE still uses its own character tables when  
 checking the case of low-valued characters, so as not to degrade performance.  
 The Unicode property information is used only for characters with higher  
 values. Even when Unicode property support is available, PCRE supports  
 case-insensitive matching only when there is a one-to-one mapping between a  
 letter's cases. There are a small number of many-to-one mappings in Unicode;  
 these are not supported by PCRE.  
 </P>  
 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>  
139  <P>  <P>
140  Philip Hazel  Philip Hazel
141  <br>  <br>
142  University Computing Service,  University Computing Service
143    <br>
144    Cambridge CB2 3QH, England.
145  <br>  <br>
 Cambridge CB2 3QG, England.  
146  </P>  </P>
147  <P>  <P>
148  Putting an actual email address here seems to have been a spam magnet, so I've  Putting an actual email address here seems to have been a spam magnet, so I've
149  taken it away. If you want to email me, use my initial and surname, separated  taken it away. If you want to email me, use my two initials, followed by the
150  by a dot, at the domain ucs.cam.ac.uk.  two digits 10, at the domain cam.ac.uk.
151  Last updated: 05 June 2006  </P>
152    <br><a name="SEC4" href="#TOC1">REVISION</a><br>
153    <P>
154    Last updated: 10 January 2012
155    <br>
156    Copyright &copy; 1997-2012 University of Cambridge.
157  <br>  <br>
 Copyright &copy; 1997-2006 University of Cambridge.  
158  <p>  <p>
159  Return to the <a href="index.html">PCRE index page</a>.  Return to the <a href="index.html">PCRE index page</a>.
160  </p>  </p>

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