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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. There is also some
29 support for certain .NET and Oniguruma syntax items, and there is an option for
30 requesting some minor changes that give better JavaScript compatibility.
31 </P>
32 <P>
33 The current implementation of PCRE (release 7.x) corresponds approximately with
34 Perl 5.10, including support for UTF-8 encoded strings and Unicode general
35 category properties. However, UTF-8 and Unicode support has to be explicitly
36 enabled; it is not the default. The Unicode tables correspond to Unicode
37 release 5.1.
38 </P>
39 <P>
40 In addition to the Perl-compatible matching function, PCRE contains an
41 alternative matching function that matches the same compiled patterns in a
42 different way. In certain circumstances, the alternative function has some
43 advantages. For a discussion of the two matching algorithms, see the
44 <a href="pcrematching.html"><b>pcrematching</b></a>
45 page.
46 </P>
47 <P>
48 PCRE is written in C and released as a C library. A number of people have
49 written wrappers and interfaces of various kinds. In particular, Google Inc.
50 have provided a comprehensive C++ wrapper. This is now included as part of the
51 PCRE distribution. The
52 <a href="pcrecpp.html"><b>pcrecpp</b></a>
53 page has details of this interface. Other people's contributions can be found
54 in the <i>Contrib</i> directory at the primary FTP site, which is:
55 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
56 </P>
57 <P>
58 Details of exactly which Perl regular expression features are and are not
59 supported by PCRE are given in separate documents. See the
60 <a href="pcrepattern.html"><b>pcrepattern</b></a>
61 and
62 <a href="pcrecompat.html"><b>pcrecompat</b></a>
63 pages. There is a syntax summary in the
64 <a href="pcresyntax.html"><b>pcresyntax</b></a>
65 page.
66 </P>
67 <P>
68 Some features of PCRE can be included, excluded, or changed when the library is
69 built. The
70 <a href="pcre_config.html"><b>pcre_config()</b></a>
71 function makes it possible for a client to discover which features are
72 available. The features themselves are described in the
73 <a href="pcrebuild.html"><b>pcrebuild</b></a>
74 page. Documentation about building PCRE for various operating systems can be
75 found in the <b>README</b> file in the source 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 are concatenated, for ease of searching. The sections are as
92 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 pcregrep description of the <b>pcregrep</b> command
102 pcrematching discussion of the two matching algorithms
103 pcrepartial details of the partial matching facility
104 pcrepattern syntax and semantics of supported regular expressions
105 pcresyntax quick syntax reference
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 sample program
110 pcrestack discussion of stack usage
111 pcretest description of the <b>pcretest</b> testing command
112 </pre>
113 In addition, in the "man" and HTML formats, there is a short page for each
114 C library function, listing its arguments and results.
115 </P>
116 <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
117 <P>
118 There are some size limitations in PCRE but it is hoped that they will never in
119 practice be relevant.
120 </P>
121 <P>
122 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
123 compiled with the default internal linkage size of 2. If you want to process
124 regular expressions that are truly enormous, you can compile PCRE with an
125 internal linkage size of 3 or 4 (see the <b>README</b> file in the source
126 distribution and the
127 <a href="pcrebuild.html"><b>pcrebuild</b></a>
128 documentation for details). In these cases the limit is substantially larger.
129 However, the speed of execution is slower.
130 </P>
131 <P>
132 All values in repeating quantifiers must be less than 65536.
133 </P>
134 <P>
135 There is no limit to the number of parenthesized subpatterns, but there can be
136 no more than 65535 capturing subpatterns.
137 </P>
138 <P>
139 The maximum length of name for a named subpattern is 32 characters, and the
140 maximum number of named subpatterns is 10000.
141 </P>
142 <P>
143 The maximum length of a subject string is the largest positive number that an
144 integer variable can hold. However, when using the traditional matching
145 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
146 This means that the available stack space may limit the size of a subject
147 string that can be processed by certain patterns. For a discussion of stack
148 issues, see the
149 <a href="pcrestack.html"><b>pcrestack</b></a>
150 documentation.
151 <a name="utf8support"></a></P>
152 <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
153 <P>
154 From release 3.3, PCRE has had some support for character strings encoded in
155 the UTF-8 format. For release 4.0 this was greatly extended to cover most
156 common requirements, and in release 5.0 additional support for Unicode general
157 category properties was added.
158 </P>
159 <P>
160 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
161 the code, and, in addition, you must call
162 <a href="pcre_compile.html"><b>pcre_compile()</b></a>
163 with the PCRE_UTF8 option flag, or the pattern must start with the sequence
164 (*UTF8). When either of these is the case, both the pattern and any subject
165 strings that are matched against it are treated as UTF-8 strings instead of
166 just strings of bytes.
167 </P>
168 <P>
169 If you compile PCRE with UTF-8 support, but do not use it at run time, the
170 library will be a bit bigger, but the additional run time overhead is limited
171 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
172 </P>
173 <P>
174 If PCRE is built with Unicode character property support (which implies UTF-8
175 support), the escape sequences \p{..}, \P{..}, and \X are supported.
176 The available properties that can be tested are limited to the general
177 category properties such as Lu for an upper case letter or Nd for a decimal
178 number, the Unicode script names such as Arabic or Han, and the derived
179 properties Any and L&. A full list is given in the
180 <a href="pcrepattern.html"><b>pcrepattern</b></a>
181 documentation. Only the short names for properties are supported. For example,
182 \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
183 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
184 compatibility with Perl 5.6. PCRE does not support this.
185 <a name="utf8strings"></a></P>
186 <br><b>
187 Validity of UTF-8 strings
188 </b><br>
189 <P>
190 When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
191 are (by default) checked for validity on entry to the relevant functions. From
192 release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
193 themselves derived from the Unicode specification. Earlier releases of PCRE
194 followed the rules of RFC 2279, which allows the full range of 31-bit values (0
195 to 0x7FFFFFFF). The current check allows only values in the range U+0 to
196 U+10FFFF, excluding U+D800 to U+DFFF.
197 </P>
198 <P>
199 The excluded code points are the "Low Surrogate Area" of Unicode, of which the
200 Unicode Standard says this: "The Low Surrogate Area does not contain any
201 character assignments, consequently no character code charts or namelists are
202 provided for this area. Surrogates are reserved for use with UTF-16 and then
203 must be used in pairs." The code points that are encoded by UTF-16 pairs are
204 available as independent code points in the UTF-8 encoding. (In other words,
205 the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
206 UTF-8.)
207 </P>
208 <P>
209 If an invalid UTF-8 string is passed to PCRE, an error return
210 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
211 your strings are valid, and therefore want to skip these checks in order to
212 improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
213 at run time, PCRE assumes that the pattern or subject it is given
214 (respectively) contains only valid UTF-8 codes. In this case, it does not
215 diagnose an invalid UTF-8 string.
216 </P>
217 <P>
218 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
219 happens depends on why the string is invalid. If the string conforms to the
220 "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
221 in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
222 test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
223 rules of RFC 2279. However, if the string does not even conform to RFC 2279,
224 the result is undefined. Your program may crash.
225 </P>
226 <P>
227 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
228 encoded in a UTF-8-like manner as per the old RFC, you can set
229 PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
230 situation, you will have to apply your own validity check.
231 </P>
232 <br><b>
233 General comments about UTF-8 mode
234 </b><br>
235 <P>
236 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
237 UTF-8 character if the value is greater than 127.
238 </P>
239 <P>
240 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
241 characters for values greater than \177.
242 </P>
243 <P>
244 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
245 bytes, for example: \x{100}{3}.
246 </P>
247 <P>
248 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
249 </P>
250 <P>
251 5. The escape sequence \C can be used to match a single byte in UTF-8 mode,
252 but its use can lead to some strange effects. This facility is not available in
253 the alternative matching function, <b>pcre_dfa_exec()</b>.
254 </P>
255 <P>
256 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
257 test characters of any code value, but the characters that PCRE recognizes as
258 digits, spaces, or word characters remain the same set as before, all with
259 values less than 256. This remains true even when PCRE includes Unicode
260 property support, because to do otherwise would slow down PCRE in many common
261 cases. If you really want to test for a wider sense of, say, "digit", you
262 must use Unicode property tests such as \p{Nd}. Note that this also applies to
263 \b, because it is defined in terms of \w and \W.
264 </P>
265 <P>
266 7. Similarly, characters that match the POSIX named character classes are all
267 low-valued characters.
268 </P>
269 <P>
270 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
271 (\h, \H, \v, and \V) do match all the appropriate Unicode characters.
272 </P>
273 <P>
274 9. Case-insensitive matching applies only to characters whose values are less
275 than 128, unless PCRE is built with Unicode property support. Even when Unicode
276 property support is available, PCRE still uses its own character tables when
277 checking the case of low-valued characters, so as not to degrade performance.
278 The Unicode property information is used only for characters with higher
279 values. Even when Unicode property support is available, PCRE supports
280 case-insensitive matching only when there is a one-to-one mapping between a
281 letter's cases. There are a small number of many-to-one mappings in Unicode;
282 these are not supported by PCRE.
283 </P>
284 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
285 <P>
286 Philip Hazel
287 <br>
288 University Computing Service
289 <br>
290 Cambridge CB2 3QH, England.
291 <br>
292 </P>
293 <P>
294 Putting an actual email address here seems to have been a spam magnet, so I've
295 taken it away. If you want to email me, use my two initials, followed by the
296 two digits 10, at the domain cam.ac.uk.
297 </P>
298 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
299 <P>
300 Last updated: 11 April 2009
301 <br>
302 Copyright &copy; 1997-2009 University of Cambridge.
303 <br>
304 <p>
305 Return to the <a href="index.html">PCRE index page</a>.
306 </p>


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