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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 <li><a name="TOC6" href="#SEC6">REVISION</a>
22 </ul>
23 <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
24 <P>
25 The PCRE library is a set of functions that implement regular expression
26 pattern matching using the same syntax and semantics as Perl, with just a few
27 differences. (Certain features that appeared in Python and PCRE before they
28 appeared in Perl are also available using the Python syntax.)
29 </P>
30 <P>
31 The current implementation of PCRE (release 7.x) corresponds approximately with
32 Perl 5.10, including support for UTF-8 encoded strings and Unicode general
33 category properties. However, UTF-8 and Unicode support has to be explicitly
34 enabled; it is not the default. The Unicode tables correspond to Unicode
35 release 5.0.0.
36 </P>
37 <P>
38 In addition to the Perl-compatible matching function, PCRE contains an
39 alternative matching function that matches the same compiled patterns in a
40 different way. In certain circumstances, the alternative function has some
41 advantages. For a discussion of the two matching algorithms, see the
42 <a href="pcrematching.html"><b>pcrematching</b></a>
43 page.
44 </P>
45 <P>
46 PCRE is written in C and released as a C library. A number of people have
47 written wrappers and interfaces of various kinds. In particular, Google Inc.
48 have provided a comprehensive C++ wrapper. This is now included as part of the
49 PCRE distribution. The
50 <a href="pcrecpp.html"><b>pcrecpp</b></a>
51 page has details of this interface. Other people's contributions can be found
52 in the <i>Contrib</i> directory at the primary FTP site, which is:
53 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
54 </P>
55 <P>
56 Details of exactly which Perl regular expression features are and are not
57 supported by PCRE are given in separate documents. See the
58 <a href="pcrepattern.html"><b>pcrepattern</b></a>
59 and
60 <a href="pcrecompat.html"><b>pcrecompat</b></a>
61 pages.
62 </P>
63 <P>
64 Some features of PCRE can be included, excluded, or changed when the library is
65 built. The
66 <a href="pcre_config.html"><b>pcre_config()</b></a>
67 function makes it possible for a client to discover which features are
68 available. The features themselves are described in the
69 <a href="pcrebuild.html"><b>pcrebuild</b></a>
70 page. Documentation about building PCRE for various operating systems can be
71 found in the <b>README</b> file in the source distribution.
72 </P>
73 <P>
74 The library contains a number of undocumented internal functions and data
75 tables that are used by more than one of the exported external functions, but
76 which are not intended for use by external callers. Their names all begin with
77 "_pcre_", which hopefully will not provoke any name clashes. In some
78 environments, it is possible to control which external symbols are exported
79 when a shared library is built, and in these cases the undocumented symbols are
80 not exported.
81 </P>
82 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
83 <P>
84 The user documentation for PCRE comprises a number of different sections. In
85 the "man" format, each of these is a separate "man page". In the HTML format,
86 each is a separate page, linked from the index page. In the plain text format,
87 all the sections are concatenated, for ease of searching. The sections are as
88 follows:
89 <pre>
90 pcre this document
91 pcreapi details of PCRE's native C API
92 pcrebuild options for building PCRE
93 pcrecallout details of the callout feature
94 pcrecompat discussion of Perl compatibility
95 pcrecpp details of the C++ wrapper
96 pcregrep description of the <b>pcregrep</b> command
97 pcrematching discussion of the two matching algorithms
98 pcrepartial details of the partial matching facility
99 pcrepattern syntax and semantics of supported regular expressions
100 pcreperform discussion of performance issues
101 pcreposix the POSIX-compatible C API
102 pcreprecompile details of saving and re-using precompiled patterns
103 pcresample discussion of the sample program
104 pcrestack discussion of stack usage
105 pcretest description of the <b>pcretest</b> testing command
106 </pre>
107 In addition, in the "man" and HTML formats, there is a short page for each
108 C library function, listing its arguments and results.
109 </P>
110 <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
111 <P>
112 There are some size limitations in PCRE but it is hoped that they will never in
113 practice be relevant.
114 </P>
115 <P>
116 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
117 compiled with the default internal linkage size of 2. If you want to process
118 regular expressions that are truly enormous, you can compile PCRE with an
119 internal linkage size of 3 or 4 (see the <b>README</b> file in the source
120 distribution and the
121 <a href="pcrebuild.html"><b>pcrebuild</b></a>
122 documentation for details). In these cases the limit is substantially larger.
123 However, the speed of execution is slower.
124 </P>
125 <P>
126 All values in repeating quantifiers must be less than 65536. The maximum
127 compiled length of subpattern with an explicit repeat count is 30000 bytes. The
128 maximum number of capturing subpatterns is 65535.
129 </P>
130 <P>
131 There is no limit to the number of parenthesized subpatterns, but there can be
132 no more than 65535 capturing subpatterns.
133 </P>
134 <P>
135 The maximum length of name for a named subpattern is 32 characters, and the
136 maximum number of named subpatterns is 10000.
137 </P>
138 <P>
139 The maximum length of a subject string is the largest positive number that an
140 integer variable can hold. However, when using the traditional matching
141 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
142 This means that the available stack space may limit the size of a subject
143 string that can be processed by certain patterns. For a discussion of stack
144 issues, see the
145 <a href="pcrestack.html"><b>pcrestack</b></a>
146 documentation.
147 <a name="utf8support"></a></P>
148 <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
149 <P>
150 From release 3.3, PCRE has had some support for character strings encoded in
151 the UTF-8 format. For release 4.0 this was greatly extended to cover most
152 common requirements, and in release 5.0 additional support for Unicode general
153 category properties was added.
154 </P>
155 <P>
156 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
157 the code, and, in addition, you must call
158 <a href="pcre_compile.html"><b>pcre_compile()</b></a>
159 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
160 subject strings that are matched against it are treated as UTF-8 strings
161 instead of just strings of bytes.
162 </P>
163 <P>
164 If you compile PCRE with UTF-8 support, but do not use it at run time, the
165 library will be a bit bigger, but the additional run time overhead is limited
166 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
167 </P>
168 <P>
169 If PCRE is built with Unicode character property support (which implies UTF-8
170 support), the escape sequences \p{..}, \P{..}, and \X are supported.
171 The available properties that can be tested are limited to the general
172 category properties such as Lu for an upper case letter or Nd for a decimal
173 number, the Unicode script names such as Arabic or Han, and the derived
174 properties Any and L&. A full list is given in the
175 <a href="pcrepattern.html"><b>pcrepattern</b></a>
176 documentation. Only the short names for properties are supported. For example,
177 \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
178 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
179 compatibility with Perl 5.6. PCRE does not support this.
180 </P>
181 <P>
182 The following comments apply when PCRE is running in UTF-8 mode:
183 </P>
184 <P>
185 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
186 are checked for validity on entry to the relevant functions. If an invalid
187 UTF-8 string is passed, an error return is given. In some situations, you may
188 already know that your strings are valid, and therefore want to skip these
189 checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
190 at compile time or at run time, PCRE assumes that the pattern or subject it
191 is given (respectively) contains only valid UTF-8 codes. In this case, it does
192 not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
193 PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
194 may crash.
195 </P>
196 <P>
197 2. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
198 UTF-8 character if the value is greater than 127.
199 </P>
200 <P>
201 3. Octal numbers up to \777 are recognized, and match two-byte UTF-8
202 characters for values greater than \177.
203 </P>
204 <P>
205 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
206 bytes, for example: \x{100}{3}.
207 </P>
208 <P>
209 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
210 </P>
211 <P>
212 6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
213 but its use can lead to some strange effects. This facility is not available in
214 the alternative matching function, <b>pcre_dfa_exec()</b>.
215 </P>
216 <P>
217 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
218 test characters of any code value, but the characters that PCRE recognizes as
219 digits, spaces, or word characters remain the same set as before, all with
220 values less than 256. This remains true even when PCRE includes Unicode
221 property support, because to do otherwise would slow down PCRE in many common
222 cases. If you really want to test for a wider sense of, say, "digit", you
223 must use Unicode property tests such as \p{Nd}.
224 </P>
225 <P>
226 8. Similarly, characters that match the POSIX named character classes are all
227 low-valued characters.
228 </P>
229 <P>
230 9. Case-insensitive matching applies only to characters whose values are less
231 than 128, unless PCRE is built with Unicode property support. Even when Unicode
232 property support is available, PCRE still uses its own character tables when
233 checking the case of low-valued characters, so as not to degrade performance.
234 The Unicode property information is used only for characters with higher
235 values. Even when Unicode property support is available, PCRE supports
236 case-insensitive matching only when there is a one-to-one mapping between a
237 letter's cases. There are a small number of many-to-one mappings in Unicode;
238 these are not supported by PCRE.
239 </P>
240 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
241 <P>
242 Philip Hazel
243 <br>
244 University Computing Service
245 <br>
246 Cambridge CB2 3QH, England.
247 <br>
248 </P>
249 <P>
250 Putting an actual email address here seems to have been a spam magnet, so I've
251 taken it away. If you want to email me, use my initial and surname, separated
252 by a dot, at the domain ucs.cam.ac.uk.
253 </P>
254 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
255 <P>
256 Last updated: 06 March 2007
257 <br>
258 Copyright &copy; 1997-2007 University of Cambridge.
259 <br>
260 <p>
261 Return to the <a href="index.html">PCRE index page</a>.
262 </p>

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