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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 pcre-config show PCRE installation configuration information
92 pcreapi details of PCRE's native C API
93 pcrebuild options for building PCRE
94 pcrecallout details of the callout feature
95 pcrecompat discussion of Perl compatibility
96 pcrecpp details of the C++ wrapper
97 pcregrep description of the <b>pcregrep</b> command
98 pcrematching discussion of the two matching algorithms
99 pcrepartial details of the partial matching facility
100 pcrepattern syntax and semantics of supported regular expressions
101 pcreperform discussion of performance issues
102 pcreposix the POSIX-compatible C API
103 pcreprecompile details of saving and re-using precompiled patterns
104 pcresample discussion of the sample program
105 pcrestack discussion of stack usage
106 pcretest description of the <b>pcretest</b> testing command
107 </pre>
108 In addition, in the "man" and HTML formats, there is a short page for each
109 C library function, listing its arguments and results.
110 </P>
111 <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
112 <P>
113 There are some size limitations in PCRE but it is hoped that they will never in
114 practice be relevant.
115 </P>
116 <P>
117 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
118 compiled with the default internal linkage size of 2. If you want to process
119 regular expressions that are truly enormous, you can compile PCRE with an
120 internal linkage size of 3 or 4 (see the <b>README</b> file in the source
121 distribution and the
122 <a href="pcrebuild.html"><b>pcrebuild</b></a>
123 documentation for details). In these cases the limit is substantially larger.
124 However, the speed of execution is slower.
125 </P>
126 <P>
127 All values in repeating quantifiers must be less than 65536. The maximum
128 compiled length of subpattern with an explicit repeat count is 30000 bytes. The
129 maximum number of capturing subpatterns is 65535.
130 </P>
131 <P>
132 There is no limit to the number of parenthesized subpatterns, but there can be
133 no more than 65535 capturing subpatterns.
134 </P>
135 <P>
136 The maximum length of name for a named subpattern is 32 characters, and the
137 maximum number of named subpatterns is 10000.
138 </P>
139 <P>
140 The maximum length of a subject string is the largest positive number that an
141 integer variable can hold. However, when using the traditional matching
142 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
143 This means that the available stack space may limit the size of a subject
144 string that can be processed by certain patterns. For a discussion of stack
145 issues, see the
146 <a href="pcrestack.html"><b>pcrestack</b></a>
147 documentation.
148 <a name="utf8support"></a></P>
149 <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
150 <P>
151 From release 3.3, PCRE has had some support for character strings encoded in
152 the UTF-8 format. For release 4.0 this was greatly extended to cover most
153 common requirements, and in release 5.0 additional support for Unicode general
154 category properties was added.
155 </P>
156 <P>
157 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
158 the code, and, in addition, you must call
159 <a href="pcre_compile.html"><b>pcre_compile()</b></a>
160 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
161 subject strings that are matched against it are treated as UTF-8 strings
162 instead of just strings of bytes.
163 </P>
164 <P>
165 If you compile PCRE with UTF-8 support, but do not use it at run time, the
166 library will be a bit bigger, but the additional run time overhead is limited
167 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
168 </P>
169 <P>
170 If PCRE is built with Unicode character property support (which implies UTF-8
171 support), the escape sequences \p{..}, \P{..}, and \X are supported.
172 The available properties that can be tested are limited to the general
173 category properties such as Lu for an upper case letter or Nd for a decimal
174 number, the Unicode script names such as Arabic or Han, and the derived
175 properties Any and L&. A full list is given in the
176 <a href="pcrepattern.html"><b>pcrepattern</b></a>
177 documentation. Only the short names for properties are supported. For example,
178 \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
179 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
180 compatibility with Perl 5.6. PCRE does not support this.
181 </P>
182 <P>
183 The following comments apply when PCRE is running in UTF-8 mode:
184 </P>
185 <P>
186 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
187 are checked for validity on entry to the relevant functions. If an invalid
188 UTF-8 string is passed, an error return is given. In some situations, you may
189 already know that your strings are valid, and therefore want to skip these
190 checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
191 at compile time or at run time, PCRE assumes that the pattern or subject it
192 is given (respectively) contains only valid UTF-8 codes. In this case, it does
193 not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
194 PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
195 may crash.
196 </P>
197 <P>
198 2. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
199 UTF-8 character if the value is greater than 127.
200 </P>
201 <P>
202 3. Octal numbers up to \777 are recognized, and match two-byte UTF-8
203 characters for values greater than \177.
204 </P>
205 <P>
206 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
207 bytes, for example: \x{100}{3}.
208 </P>
209 <P>
210 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
211 </P>
212 <P>
213 6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
214 but its use can lead to some strange effects. This facility is not available in
215 the alternative matching function, <b>pcre_dfa_exec()</b>.
216 </P>
217 <P>
218 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
219 test characters of any code value, but the characters that PCRE recognizes as
220 digits, spaces, or word characters remain the same set as before, all with
221 values less than 256. This remains true even when PCRE includes Unicode
222 property support, because to do otherwise would slow down PCRE in many common
223 cases. If you really want to test for a wider sense of, say, "digit", you
224 must use Unicode property tests such as \p{Nd}.
225 </P>
226 <P>
227 8. Similarly, characters that match the POSIX named character classes are all
228 low-valued characters.
229 </P>
230 <P>
231 9. Case-insensitive matching applies only to characters whose values are less
232 than 128, unless PCRE is built with Unicode property support. Even when Unicode
233 property support is available, PCRE still uses its own character tables when
234 checking the case of low-valued characters, so as not to degrade performance.
235 The Unicode property information is used only for characters with higher
236 values. Even when Unicode property support is available, PCRE supports
237 case-insensitive matching only when there is a one-to-one mapping between a
238 letter's cases. There are a small number of many-to-one mappings in Unicode;
239 these are not supported by PCRE.
240 </P>
241 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
242 <P>
243 Philip Hazel
244 <br>
245 University Computing Service
246 <br>
247 Cambridge CB2 3QH, England.
248 <br>
249 </P>
250 <P>
251 Putting an actual email address here seems to have been a spam magnet, so I've
252 taken it away. If you want to email me, use my two initials, followed by the
253 two digits 10, at the domain cam.ac.uk.
254 </P>
255 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
256 <P>
257 Last updated: 18 April 2007
258 <br>
259 Copyright &copy; 1997-2007 University of Cambridge.
260 <br>
261 <p>
262 Return to the <a href="index.html">PCRE index page</a>.
263 </p>

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