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1 <html>
2 <head>
3 <title>pcre specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <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 <ul>
16 <li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
17 <li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
18 <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
19 <li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
20 <li><a name="TOC5" href="#SEC5">AUTHOR</a>
21 </ul>
22 <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
23 <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 differences. (Certain features that appeared in Python and PCRE before they
27 appeared in Perl are also available using the Python syntax.)
28 </P>
29 <P>
30 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 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 PCRE is written in C and released as a C library. A number of people have
46 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 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
53 </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 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 </P>
72 <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 "_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 </P>
81 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
82 <P>
83 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 <pre>
89 pcre this document
90 pcreapi details of PCRE's native C API
91 pcrebuild options for building PCRE
92 pcrecallout details of the callout feature
93 pcrecompat discussion of Perl compatibility
94 pcrecpp details of the C++ wrapper
95 pcregrep description of the <b>pcregrep</b> command
96 pcrematching discussion of the two matching algorithms
97 pcrepartial details of the partial matching facility
98 pcrepattern syntax and semantics of supported regular expressions
99 pcreperform discussion of performance issues
100 pcreposix the POSIX-compatible C API
101 pcreprecompile details of saving and re-using precompiled patterns
102 pcresample discussion of the sample program
103 pcrestack discussion of stack usage
104 pcretest description of the <b>pcretest</b> testing command
105 </pre>
106 In addition, in the "man" and HTML formats, there is a short page for each
107 C library function, listing its arguments and results.
108 </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 documentation for details). In these cases the limit is substantially larger.
122 However, the speed of execution is slower.
123 </P>
124 <P>
125 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 </P>
129 <P>
130 There is no limit to the number of parenthesized subpatterns, but there can be
131 no more than 65535 capturing subpatterns.
132 </P>
133 <P>
134 The maximum length of name for a named subpattern is 32 characters, and the
135 maximum number of named subpatterns is 10000.
136 </P>
137 <P>
138 The maximum length of a subject string is the largest positive number that an
139 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 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 <a name="utf8support"></a></P>
147 <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
148 <P>
149 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 </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 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
166 </P>
167 <P>
168 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 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 <a href="pcrepattern.html"><b>pcrepattern</b></a>
175 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 </P>
180 <P>
181 The following comments apply when PCRE is running in UTF-8 mode:
182 </P>
183 <P>
184 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 </P>
195 <P>
196 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 </P>
199 <P>
200 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 bytes, for example: \x{100}{3}.
206 </P>
207 <P>
208 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
209 </P>
210 <P>
211 6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
212 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 </P>
215 <P>
216 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
217 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 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 </P>
224 <P>
225 8. Similarly, characters that match the POSIX named character classes are all
226 low-valued characters.
227 </P>
228 <P>
229 9. Case-insensitive matching applies only to characters whose values are less
230 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 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 </P>
239 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
240 <P>
241 Philip Hazel
242 <br>
243 University Computing Service,
244 <br>
245 Cambridge CB2 3QH, England.
246 </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 Last updated: 23 November 2006
252 <br>
253 Copyright &copy; 1997-2006 University of Cambridge.
254 <p>
255 Return to the <a href="index.html">PCRE index page</a>.
256 </p>

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