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1 nigel 63 <html>
2     <head>
3     <title>pcre specification</title>
4     </head>
5     <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 nigel 75 <h1>pcre man page</h1>
7     <p>
8     Return to the <a href="index.html">PCRE index page</a>.
9     </p>
10     <p>
11     This page is part of the PCRE HTML documentation. It was generated automatically
12     from the original man page. If there is any nonsense in it, please consult the
13     man page, in case the conversion went wrong.
14     <br>
15 nigel 63 <ul>
16 nigel 75 <li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
17 nigel 63 <li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
18     <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
19 nigel 75 <li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
20 nigel 63 <li><a name="TOC5" href="#SEC5">AUTHOR</a>
21     </ul>
22 nigel 75 <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
23 nigel 63 <P>
24     The PCRE library is a set of functions that implement regular expression
25     pattern matching using the same syntax and semantics as Perl, with just a few
26 nigel 93 differences. (Certain features that appeared in Python and PCRE before they
27     appeared in Perl are also available using the Python syntax.)
28 nigel 63 </P>
29     <P>
30 nigel 93 The current implementation of PCRE (release 7.x) corresponds approximately with
31     Perl 5.10, including support for UTF-8 encoded strings and Unicode general
32     category properties. However, UTF-8 and Unicode support has to be explicitly
33     enabled; it is not the default. The Unicode tables correspond to Unicode
34     release 5.0.0.
35     </P>
36     <P>
37     In addition to the Perl-compatible matching function, PCRE contains an
38 nigel 77 alternative matching function that matches the same compiled patterns in a
39     different way. In certain circumstances, the alternative function has some
40     advantages. For a discussion of the two matching algorithms, see the
41     <a href="pcrematching.html"><b>pcrematching</b></a>
42     page.
43     </P>
44     <P>
45 nigel 75 PCRE is written in C and released as a C library. A number of people have
46 nigel 77 written wrappers and interfaces of various kinds. In particular, Google Inc.
47     have provided a comprehensive C++ wrapper. This is now included as part of the
48     PCRE distribution. The
49     <a href="pcrecpp.html"><b>pcrecpp</b></a>
50     page has details of this interface. Other people's contributions can be found
51     in the <i>Contrib</i> directory at the primary FTP site, which is:
52 nigel 75 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
53 nigel 63 </P>
54     <P>
55     Details of exactly which Perl regular expression features are and are not
56     supported by PCRE are given in separate documents. See the
57     <a href="pcrepattern.html"><b>pcrepattern</b></a>
58     and
59     <a href="pcrecompat.html"><b>pcrecompat</b></a>
60     pages.
61     </P>
62     <P>
63     Some features of PCRE can be included, excluded, or changed when the library is
64     built. The
65     <a href="pcre_config.html"><b>pcre_config()</b></a>
66     function makes it possible for a client to discover which features are
67 nigel 75 available. The features themselves are described in the
68     <a href="pcrebuild.html"><b>pcrebuild</b></a>
69     page. Documentation about building PCRE for various operating systems can be
70     found in the <b>README</b> file in the source distribution.
71 nigel 63 </P>
72 nigel 77 <P>
73     The library contains a number of undocumented internal functions and data
74     tables that are used by more than one of the exported external functions, but
75     which are not intended for use by external callers. Their names all begin with
76 nigel 83 "_pcre_", which hopefully will not provoke any name clashes. In some
77     environments, it is possible to control which external symbols are exported
78     when a shared library is built, and in these cases the undocumented symbols are
79     not exported.
80 nigel 77 </P>
81 nigel 63 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
82     <P>
83 nigel 75 The user documentation for PCRE comprises a number of different sections. In
84     the "man" format, each of these is a separate "man page". In the HTML format,
85     each is a separate page, linked from the index page. In the plain text format,
86     all the sections are concatenated, for ease of searching. The sections are as
87     follows:
88 nigel 63 <pre>
89     pcre this document
90 nigel 77 pcreapi details of PCRE's native C API
91 nigel 63 pcrebuild options for building PCRE
92     pcrecallout details of the callout feature
93     pcrecompat discussion of Perl compatibility
94 nigel 77 pcrecpp details of the C++ wrapper
95 nigel 63 pcregrep description of the <b>pcregrep</b> command
96 nigel 77 pcrematching discussion of the two matching algorithms
97 nigel 75 pcrepartial details of the partial matching facility
98     pcrepattern syntax and semantics of supported regular expressions
99 nigel 63 pcreperform discussion of performance issues
100 nigel 77 pcreposix the POSIX-compatible C API
101 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
102 nigel 63 pcresample discussion of the sample program
103 nigel 91 pcrestack discussion of stack usage
104 nigel 75 pcretest description of the <b>pcretest</b> testing command
105     </pre>
106 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
107 nigel 77 C library function, listing its arguments and results.
108 nigel 63 </P>
109     <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
110     <P>
111     There are some size limitations in PCRE but it is hoped that they will never in
112     practice be relevant.
113     </P>
114     <P>
115     The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
116     compiled with the default internal linkage size of 2. If you want to process
117     regular expressions that are truly enormous, you can compile PCRE with an
118     internal linkage size of 3 or 4 (see the <b>README</b> file in the source
119     distribution and the
120     <a href="pcrebuild.html"><b>pcrebuild</b></a>
121 nigel 75 documentation for details). In these cases the limit is substantially larger.
122 nigel 93 However, the speed of execution is slower.
123 nigel 63 </P>
124     <P>
125 nigel 91 All values in repeating quantifiers must be less than 65536. The maximum
126     compiled length of subpattern with an explicit repeat count is 30000 bytes. The
127     maximum number of capturing subpatterns is 65535.
128 nigel 63 </P>
129     <P>
130 nigel 93 There is no limit to the number of parenthesized subpatterns, but there can be
131     no more than 65535 capturing subpatterns.
132 nigel 63 </P>
133     <P>
134 nigel 93 The maximum length of name for a named subpattern is 32 characters, and the
135     maximum number of named subpatterns is 10000.
136 nigel 91 </P>
137     <P>
138 nigel 63 The maximum length of a subject string is the largest positive number that an
139 nigel 77 integer variable can hold. However, when using the traditional matching
140     function, PCRE uses recursion to handle subpatterns and indefinite repetition.
141     This means that the available stack space may limit the size of a subject
142 nigel 91 string that can be processed by certain patterns. For a discussion of stack
143     issues, see the
144     <a href="pcrestack.html"><b>pcrestack</b></a>
145     documentation.
146 nigel 75 <a name="utf8support"></a></P>
147     <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
148 nigel 63 <P>
149 nigel 75 From release 3.3, PCRE has had some support for character strings encoded in
150     the UTF-8 format. For release 4.0 this was greatly extended to cover most
151     common requirements, and in release 5.0 additional support for Unicode general
152     category properties was added.
153 nigel 63 </P>
154     <P>
155     In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
156     the code, and, in addition, you must call
157     <a href="pcre_compile.html"><b>pcre_compile()</b></a>
158     with the PCRE_UTF8 option flag. When you do this, both the pattern and any
159     subject strings that are matched against it are treated as UTF-8 strings
160     instead of just strings of bytes.
161     </P>
162     <P>
163     If you compile PCRE with UTF-8 support, but do not use it at run time, the
164     library will be a bit bigger, but the additional run time overhead is limited
165 nigel 93 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
166 nigel 63 </P>
167     <P>
168 nigel 75 If PCRE is built with Unicode character property support (which implies UTF-8
169     support), the escape sequences \p{..}, \P{..}, and \X are supported.
170     The available properties that can be tested are limited to the general
171     category properties such as Lu for an upper case letter or Nd for a decimal
172 nigel 87 number, the Unicode script names such as Arabic or Han, and the derived
173     properties Any and L&. A full list is given in the
174 nigel 75 <a href="pcrepattern.html"><b>pcrepattern</b></a>
175 nigel 87 documentation. Only the short names for properties are supported. For example,
176     \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
177     Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
178     compatibility with Perl 5.6. PCRE does not support this.
179 nigel 75 </P>
180     <P>
181 nigel 63 The following comments apply when PCRE is running in UTF-8 mode:
182     </P>
183     <P>
184 nigel 71 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
185     are checked for validity on entry to the relevant functions. If an invalid
186     UTF-8 string is passed, an error return is given. In some situations, you may
187     already know that your strings are valid, and therefore want to skip these
188     checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
189     at compile time or at run time, PCRE assumes that the pattern or subject it
190     is given (respectively) contains only valid UTF-8 codes. In this case, it does
191     not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
192     PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
193     may crash.
194 nigel 63 </P>
195     <P>
196 nigel 87 2. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
197     UTF-8 character if the value is greater than 127.
198 nigel 63 </P>
199     <P>
200 nigel 91 3. Octal numbers up to \777 are recognized, and match two-byte UTF-8
201     characters for values greater than \177.
202     </P>
203     <P>
204     4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
205 nigel 63 bytes, for example: \x{100}{3}.
206     </P>
207     <P>
208 nigel 91 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
209 nigel 63 </P>
210     <P>
211 nigel 91 6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
212 nigel 77 but its use can lead to some strange effects. This facility is not available in
213     the alternative matching function, <b>pcre_dfa_exec()</b>.
214 nigel 63 </P>
215     <P>
216 nigel 91 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
217 nigel 63 test characters of any code value, but the characters that PCRE recognizes as
218     digits, spaces, or word characters remain the same set as before, all with
219 nigel 75 values less than 256. This remains true even when PCRE includes Unicode
220     property support, because to do otherwise would slow down PCRE in many common
221     cases. If you really want to test for a wider sense of, say, "digit", you
222     must use Unicode property tests such as \p{Nd}.
223 nigel 63 </P>
224     <P>
225 nigel 91 8. Similarly, characters that match the POSIX named character classes are all
226 nigel 75 low-valued characters.
227 nigel 63 </P>
228     <P>
229 nigel 91 9. Case-insensitive matching applies only to characters whose values are less
230 nigel 75 than 128, unless PCRE is built with Unicode property support. Even when Unicode
231     property support is available, PCRE still uses its own character tables when
232     checking the case of low-valued characters, so as not to degrade performance.
233     The Unicode property information is used only for characters with higher
234 nigel 87 values. Even when Unicode property support is available, PCRE supports
235     case-insensitive matching only when there is a one-to-one mapping between a
236     letter's cases. There are a small number of many-to-one mappings in Unicode;
237     these are not supported by PCRE.
238 nigel 63 </P>
239     <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
240     <P>
241 nigel 77 Philip Hazel
242 nigel 63 <br>
243     University Computing Service,
244     <br>
245 nigel 93 Cambridge CB2 3QH, England.
246 nigel 77 </P>
247     <P>
248     Putting an actual email address here seems to have been a spam magnet, so I've
249     taken it away. If you want to email me, use my initial and surname, separated
250     by a dot, at the domain ucs.cam.ac.uk.
251 nigel 93 Last updated: 23 November 2006
252 nigel 63 <br>
253 nigel 87 Copyright &copy; 1997-2006 University of Cambridge.
254 nigel 75 <p>
255     Return to the <a href="index.html">PCRE index page</a>.
256     </p>

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