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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. Some features that appeared in Python and PCRE before they
28 appeared in Perl are also available using the Python syntax, there is some
29 support for one or two .NET and Oniguruma syntax items, and there is an option
30 for requesting some minor changes that give better JavaScript compatibility.
31 </P>
32 <P>
33 The current implementation of PCRE corresponds approximately with Perl 5.10,
34 including support for UTF-8 encoded strings and Unicode general category
35 properties. However, UTF-8 and Unicode support has to be explicitly enabled; it
36 is not the default. The Unicode tables correspond to Unicode release 5.1.
37 </P>
38 <P>
39 In addition to the Perl-compatible matching function, PCRE contains an
40 alternative function that matches the same compiled patterns in a different
41 way. In certain circumstances, the alternative function has some advantages.
42 For a discussion of the two matching algorithms, see the
43 <a href="pcrematching.html"><b>pcrematching</b></a>
44 page.
45 </P>
46 <P>
47 PCRE is written in C and released as a C library. A number of people have
48 written wrappers and interfaces of various kinds. In particular, Google Inc.
49 have provided a comprehensive C++ wrapper. This is now included as part of the
50 PCRE distribution. The
51 <a href="pcrecpp.html"><b>pcrecpp</b></a>
52 page has details of this interface. Other people's contributions can be found
53 in the <i>Contrib</i> directory at the primary FTP site, which is:
54 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
55 </P>
56 <P>
57 Details of exactly which Perl regular expression features are and are not
58 supported by PCRE are given in separate documents. See the
59 <a href="pcrepattern.html"><b>pcrepattern</b></a>
60 and
61 <a href="pcrecompat.html"><b>pcrecompat</b></a>
62 pages. There is a syntax summary in the
63 <a href="pcresyntax.html"><b>pcresyntax</b></a>
64 page.
65 </P>
66 <P>
67 Some features of PCRE can be included, excluded, or changed when the library is
68 built. The
69 <a href="pcre_config.html"><b>pcre_config()</b></a>
70 function makes it possible for a client to discover which features are
71 available. The features themselves are described in the
72 <a href="pcrebuild.html"><b>pcrebuild</b></a>
73 page. Documentation about building PCRE for various operating systems can be
74 found in the <b>README</b> and <b>NON-UNIX-USE</b> files in the source
75 distribution.
76 </P>
77 <P>
78 The library contains a number of undocumented internal functions and data
79 tables that are used by more than one of the exported external functions, but
80 which are not intended for use by external callers. Their names all begin with
81 "_pcre_", which hopefully will not provoke any name clashes. In some
82 environments, it is possible to control which external symbols are exported
83 when a shared library is built, and in these cases the undocumented symbols are
84 not exported.
85 </P>
86 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
87 <P>
88 The user documentation for PCRE comprises a number of different sections. In
89 the "man" format, each of these is a separate "man page". In the HTML format,
90 each is a separate page, linked from the index page. In the plain text format,
91 all the sections, except the <b>pcredemo</b> section, are concatenated, for ease
92 of searching. The sections are as follows:
93 <pre>
94 pcre this document
95 pcre-config show PCRE installation configuration information
96 pcreapi details of PCRE's native C API
97 pcrebuild options for building PCRE
98 pcrecallout details of the callout feature
99 pcrecompat discussion of Perl compatibility
100 pcrecpp details of the C++ wrapper
101 pcredemo a demonstration C program that uses PCRE
102 pcregrep description of the <b>pcregrep</b> command
103 pcrematching discussion of the two matching algorithms
104 pcrepartial details of the partial matching facility
105 pcrepattern syntax and semantics of supported regular expressions
106 pcreperform discussion of performance issues
107 pcreposix the POSIX-compatible C API
108 pcreprecompile details of saving and re-using precompiled patterns
109 pcresample discussion of the pcredemo program
110 pcrestack discussion of stack usage
111 pcresyntax quick syntax reference
112 pcretest description of the <b>pcretest</b> testing command
113 </pre>
114 In addition, in the "man" and HTML formats, there is a short page for each
115 C library function, listing its arguments and results.
116 </P>
117 <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
118 <P>
119 There are some size limitations in PCRE but it is hoped that they will never in
120 practice be relevant.
121 </P>
122 <P>
123 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
124 compiled with the default internal linkage size of 2. If you want to process
125 regular expressions that are truly enormous, you can compile PCRE with an
126 internal linkage size of 3 or 4 (see the <b>README</b> file in the source
127 distribution and the
128 <a href="pcrebuild.html"><b>pcrebuild</b></a>
129 documentation for details). In these cases the limit is substantially larger.
130 However, the speed of execution is slower.
131 </P>
132 <P>
133 All values in repeating quantifiers must be less than 65536.
134 </P>
135 <P>
136 There is no limit to the number of parenthesized subpatterns, but there can be
137 no more than 65535 capturing subpatterns.
138 </P>
139 <P>
140 The maximum length of name for a named subpattern is 32 characters, and the
141 maximum number of named subpatterns is 10000.
142 </P>
143 <P>
144 The maximum length of a subject string is the largest positive number that an
145 integer variable can hold. However, when using the traditional matching
146 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
147 This means that the available stack space may limit the size of a subject
148 string that can be processed by certain patterns. For a discussion of stack
149 issues, see the
150 <a href="pcrestack.html"><b>pcrestack</b></a>
151 documentation.
152 <a name="utf8support"></a></P>
153 <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
154 <P>
155 From release 3.3, PCRE has had some support for character strings encoded in
156 the UTF-8 format. For release 4.0 this was greatly extended to cover most
157 common requirements, and in release 5.0 additional support for Unicode general
158 category properties was added.
159 </P>
160 <P>
161 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
162 the code, and, in addition, you must call
163 <a href="pcre_compile.html"><b>pcre_compile()</b></a>
164 with the PCRE_UTF8 option flag, or the pattern must start with the sequence
165 (*UTF8). When either of these is the case, both the pattern and any subject
166 strings that are matched against it are treated as UTF-8 strings instead of
167 strings of 1-byte characters.
168 </P>
169 <P>
170 If you compile PCRE with UTF-8 support, but do not use it at run time, the
171 library will be a bit bigger, but the additional run time overhead is limited
172 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
173 </P>
174 <P>
175 If PCRE is built with Unicode character property support (which implies UTF-8
176 support), the escape sequences \p{..}, \P{..}, and \X are supported.
177 The available properties that can be tested are limited to the general
178 category properties such as Lu for an upper case letter or Nd for a decimal
179 number, the Unicode script names such as Arabic or Han, and the derived
180 properties Any and L&. A full list is given in the
181 <a href="pcrepattern.html"><b>pcrepattern</b></a>
182 documentation. Only the short names for properties are supported. For example,
183 \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
184 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
185 compatibility with Perl 5.6. PCRE does not support this.
186 <a name="utf8strings"></a></P>
187 <br><b>
188 Validity of UTF-8 strings
189 </b><br>
190 <P>
191 When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
192 are (by default) checked for validity on entry to the relevant functions. From
193 release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
194 themselves derived from the Unicode specification. Earlier releases of PCRE
195 followed the rules of RFC 2279, which allows the full range of 31-bit values (0
196 to 0x7FFFFFFF). The current check allows only values in the range U+0 to
197 U+10FFFF, excluding U+D800 to U+DFFF.
198 </P>
199 <P>
200 The excluded code points are the "Low Surrogate Area" of Unicode, of which the
201 Unicode Standard says this: "The Low Surrogate Area does not contain any
202 character assignments, consequently no character code charts or namelists are
203 provided for this area. Surrogates are reserved for use with UTF-16 and then
204 must be used in pairs." The code points that are encoded by UTF-16 pairs are
205 available as independent code points in the UTF-8 encoding. (In other words,
206 the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
207 UTF-8.)
208 </P>
209 <P>
210 If an invalid UTF-8 string is passed to PCRE, an error return
211 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
212 your strings are valid, and therefore want to skip these checks in order to
213 improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
214 at run time, PCRE assumes that the pattern or subject it is given
215 (respectively) contains only valid UTF-8 codes. In this case, it does not
216 diagnose an invalid UTF-8 string.
217 </P>
218 <P>
219 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
220 happens depends on why the string is invalid. If the string conforms to the
221 "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
222 in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
223 test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
224 rules of RFC 2279. However, if the string does not even conform to RFC 2279,
225 the result is undefined. Your program may crash.
226 </P>
227 <P>
228 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
229 encoded in a UTF-8-like manner as per the old RFC, you can set
230 PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
231 situation, you will have to apply your own validity check.
232 </P>
233 <br><b>
234 General comments about UTF-8 mode
235 </b><br>
236 <P>
237 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
238 UTF-8 character if the value is greater than 127.
239 </P>
240 <P>
241 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
242 characters for values greater than \177.
243 </P>
244 <P>
245 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
246 bytes, for example: \x{100}{3}.
247 </P>
248 <P>
249 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
250 </P>
251 <P>
252 5. The escape sequence \C can be used to match a single byte in UTF-8 mode,
253 but its use can lead to some strange effects. This facility is not available in
254 the alternative matching function, <b>pcre_dfa_exec()</b>.
255 </P>
256 <P>
257 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
258 test characters of any code value, but the characters that PCRE recognizes as
259 digits, spaces, or word characters remain the same set as before, all with
260 values less than 256. This remains true even when PCRE includes Unicode
261 property support, because to do otherwise would slow down PCRE in many common
262 cases. If you really want to test for a wider sense of, say, "digit", you
263 must use Unicode property tests such as \p{Nd}. Note that this also applies to
264 \b, because it is defined in terms of \w and \W.
265 </P>
266 <P>
267 7. Similarly, characters that match the POSIX named character classes are all
268 low-valued characters.
269 </P>
270 <P>
271 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
272 (\h, \H, \v, and \V) do match all the appropriate Unicode characters.
273 </P>
274 <P>
275 9. Case-insensitive matching applies only to characters whose values are less
276 than 128, unless PCRE is built with Unicode property support. Even when Unicode
277 property support is available, PCRE still uses its own character tables when
278 checking the case of low-valued characters, so as not to degrade performance.
279 The Unicode property information is used only for characters with higher
280 values. Even when Unicode property support is available, PCRE supports
281 case-insensitive matching only when there is a one-to-one mapping between a
282 letter's cases. There are a small number of many-to-one mappings in Unicode;
283 these are not supported by PCRE.
284 </P>
285 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
286 <P>
287 Philip Hazel
288 <br>
289 University Computing Service
290 <br>
291 Cambridge CB2 3QH, England.
292 <br>
293 </P>
294 <P>
295 Putting an actual email address here seems to have been a spam magnet, so I've
296 taken it away. If you want to email me, use my two initials, followed by the
297 two digits 10, at the domain cam.ac.uk.
298 </P>
299 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
300 <P>
301 Last updated: 28 September 2009
302 <br>
303 Copyright &copy; 1997-2009 University of Cambridge.
304 <br>
305 <p>
306 Return to the <a href="index.html">PCRE index page</a>.
307 </p>

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