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1 nigel 63 <html>
2     <head>
3     <title>pcre specification</title>
4     </head>
5     <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 nigel 75 <h1>pcre man page</h1>
7     <p>
8     Return to the <a href="index.html">PCRE index page</a>.
9     </p>
10 ph10 111 <p>
11 nigel 75 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 ph10 111 <br>
15 nigel 63 <ul>
16 nigel 75 <li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
17 nigel 63 <li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
18     <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
19 nigel 75 <li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
20 nigel 63 <li><a name="TOC5" href="#SEC5">AUTHOR</a>
21 ph10 99 <li><a name="TOC6" href="#SEC6">REVISION</a>
22 nigel 63 </ul>
23 nigel 75 <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
24 nigel 63 <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 ph10 461 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 nigel 63 </P>
32     <P>
33 ph10 518 The current implementation of PCRE corresponds approximately with Perl
34     5.10/5.11, 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 5.2.0.
38 nigel 93 </P>
39     <P>
40     In addition to the Perl-compatible matching function, PCRE contains an
41 ph10 461 alternative function that matches the same compiled patterns in a different
42     way. In certain circumstances, the alternative function has some advantages.
43     For a discussion of the two matching algorithms, see the
44 nigel 77 <a href="pcrematching.html"><b>pcrematching</b></a>
45     page.
46     </P>
47     <P>
48 nigel 75 PCRE is written in C and released as a C library. A number of people have
49 nigel 77 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 nigel 75 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
56 nigel 63 </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 ph10 208 pages. There is a syntax summary in the
64     <a href="pcresyntax.html"><b>pcresyntax</b></a>
65     page.
66 nigel 63 </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 nigel 75 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 ph10 461 found in the <b>README</b> and <b>NON-UNIX-USE</b> files in the source
76     distribution.
77 nigel 63 </P>
78 nigel 77 <P>
79     The library contains a number of undocumented internal functions and data
80     tables that are used by more than one of the exported external functions, but
81     which are not intended for use by external callers. Their names all begin with
82 nigel 83 "_pcre_", which hopefully will not provoke any name clashes. In some
83     environments, it is possible to control which external symbols are exported
84     when a shared library is built, and in these cases the undocumented symbols are
85     not exported.
86 nigel 77 </P>
87 nigel 63 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
88     <P>
89 nigel 75 The user documentation for PCRE comprises a number of different sections. In
90     the "man" format, each of these is a separate "man page". In the HTML format,
91     each is a separate page, linked from the index page. In the plain text format,
92 ph10 429 all the sections, except the <b>pcredemo</b> section, are concatenated, for ease
93     of searching. The sections are as follows:
94 nigel 63 <pre>
95     pcre this document
96 ph10 153 pcre-config show PCRE installation configuration information
97 nigel 77 pcreapi details of PCRE's native C API
98 nigel 63 pcrebuild options for building PCRE
99     pcrecallout details of the callout feature
100     pcrecompat discussion of Perl compatibility
101 nigel 77 pcrecpp details of the C++ wrapper
102 ph10 429 pcredemo a demonstration C program that uses PCRE
103 nigel 63 pcregrep description of the <b>pcregrep</b> command
104 nigel 77 pcrematching discussion of the two matching algorithms
105 nigel 75 pcrepartial details of the partial matching facility
106     pcrepattern syntax and semantics of supported regular expressions
107 nigel 63 pcreperform discussion of performance issues
108 nigel 77 pcreposix the POSIX-compatible C API
109 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
110 ph10 429 pcresample discussion of the pcredemo program
111 nigel 91 pcrestack discussion of stack usage
112 ph10 461 pcresyntax quick syntax reference
113 nigel 75 pcretest description of the <b>pcretest</b> testing command
114     </pre>
115 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
116 nigel 77 C library function, listing its arguments and results.
117 nigel 63 </P>
118     <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
119     <P>
120     There are some size limitations in PCRE but it is hoped that they will never in
121     practice be relevant.
122     </P>
123     <P>
124     The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
125     compiled with the default internal linkage size of 2. If you want to process
126     regular expressions that are truly enormous, you can compile PCRE with an
127     internal linkage size of 3 or 4 (see the <b>README</b> file in the source
128     distribution and the
129     <a href="pcrebuild.html"><b>pcrebuild</b></a>
130 nigel 75 documentation for details). In these cases the limit is substantially larger.
131 nigel 93 However, the speed of execution is slower.
132 nigel 63 </P>
133     <P>
134 ph10 208 All values in repeating quantifiers must be less than 65536.
135 nigel 63 </P>
136     <P>
137 nigel 93 There is no limit to the number of parenthesized subpatterns, but there can be
138     no more than 65535 capturing subpatterns.
139 nigel 63 </P>
140     <P>
141 nigel 93 The maximum length of name for a named subpattern is 32 characters, and the
142     maximum number of named subpatterns is 10000.
143 nigel 91 </P>
144     <P>
145 nigel 63 The maximum length of a subject string is the largest positive number that an
146 nigel 77 integer variable can hold. However, when using the traditional matching
147     function, PCRE uses recursion to handle subpatterns and indefinite repetition.
148     This means that the available stack space may limit the size of a subject
149 nigel 91 string that can be processed by certain patterns. For a discussion of stack
150     issues, see the
151     <a href="pcrestack.html"><b>pcrestack</b></a>
152     documentation.
153 nigel 75 <a name="utf8support"></a></P>
154     <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
155 nigel 63 <P>
156 nigel 75 From release 3.3, PCRE has had some support for character strings encoded in
157     the UTF-8 format. For release 4.0 this was greatly extended to cover most
158     common requirements, and in release 5.0 additional support for Unicode general
159     category properties was added.
160 nigel 63 </P>
161     <P>
162     In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
163     the code, and, in addition, you must call
164     <a href="pcre_compile.html"><b>pcre_compile()</b></a>
165 ph10 416 with the PCRE_UTF8 option flag, or the pattern must start with the sequence
166     (*UTF8). When either of these is the case, both the pattern and any subject
167     strings that are matched against it are treated as UTF-8 strings instead of
168 ph10 461 strings of 1-byte characters.
169 nigel 63 </P>
170     <P>
171     If you compile PCRE with UTF-8 support, but do not use it at run time, the
172     library will be a bit bigger, but the additional run time overhead is limited
173 nigel 93 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
174 nigel 63 </P>
175     <P>
176 nigel 75 If PCRE is built with Unicode character property support (which implies UTF-8
177     support), the escape sequences \p{..}, \P{..}, and \X are supported.
178     The available properties that can be tested are limited to the general
179     category properties such as Lu for an upper case letter or Nd for a decimal
180 nigel 87 number, the Unicode script names such as Arabic or Han, and the derived
181     properties Any and L&. A full list is given in the
182 nigel 75 <a href="pcrepattern.html"><b>pcrepattern</b></a>
183 nigel 87 documentation. Only the short names for properties are supported. For example,
184     \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
185     Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
186     compatibility with Perl 5.6. PCRE does not support this.
187 ph10 211 <a name="utf8strings"></a></P>
188     <br><b>
189     Validity of UTF-8 strings
190     </b><br>
191     <P>
192     When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
193     are (by default) checked for validity on entry to the relevant functions. From
194     release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
195     themselves derived from the Unicode specification. Earlier releases of PCRE
196     followed the rules of RFC 2279, which allows the full range of 31-bit values (0
197     to 0x7FFFFFFF). The current check allows only values in the range U+0 to
198     U+10FFFF, excluding U+D800 to U+DFFF.
199 nigel 75 </P>
200     <P>
201 ph10 211 The excluded code points are the "Low Surrogate Area" of Unicode, of which the
202     Unicode Standard says this: "The Low Surrogate Area does not contain any
203     character assignments, consequently no character code charts or namelists are
204     provided for this area. Surrogates are reserved for use with UTF-16 and then
205     must be used in pairs." The code points that are encoded by UTF-16 pairs are
206     available as independent code points in the UTF-8 encoding. (In other words,
207     the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
208     UTF-8.)
209 nigel 63 </P>
210     <P>
211 ph10 211 If an invalid UTF-8 string is passed to PCRE, an error return
212     (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
213     your strings are valid, and therefore want to skip these checks in order to
214     improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
215     at run time, PCRE assumes that the pattern or subject it is given
216     (respectively) contains only valid UTF-8 codes. In this case, it does not
217     diagnose an invalid UTF-8 string.
218 nigel 63 </P>
219     <P>
220 ph10 211 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
221     happens depends on why the string is invalid. If the string conforms to the
222     "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
223     in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
224     test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
225     rules of RFC 2279. However, if the string does not even conform to RFC 2279,
226     the result is undefined. Your program may crash.
227     </P>
228     <P>
229     If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
230     encoded in a UTF-8-like manner as per the old RFC, you can set
231     PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
232     situation, you will have to apply your own validity check.
233     </P>
234     <br><b>
235     General comments about UTF-8 mode
236     </b><br>
237     <P>
238     1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
239 nigel 87 UTF-8 character if the value is greater than 127.
240 nigel 63 </P>
241     <P>
242 ph10 211 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
243 nigel 91 characters for values greater than \177.
244     </P>
245     <P>
246 ph10 211 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
247 nigel 63 bytes, for example: \x{100}{3}.
248     </P>
249     <P>
250 ph10 211 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
251 nigel 63 </P>
252     <P>
253 ph10 211 5. The escape sequence \C can be used to match a single byte in UTF-8 mode,
254 nigel 77 but its use can lead to some strange effects. This facility is not available in
255     the alternative matching function, <b>pcre_dfa_exec()</b>.
256 nigel 63 </P>
257     <P>
258 ph10 211 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
259 ph10 518 test characters of any code value, but, by default, the characters that PCRE
260     recognizes as digits, spaces, or word characters remain the same set as before,
261     all with values less than 256. This remains true even when PCRE is built to
262     include Unicode property support, because to do otherwise would slow down PCRE
263     in many common cases. Note that this also applies to \b, because it is defined
264     in terms of \w and \W. If you really want to test for a wider sense of, say,
265     "digit", you can use explicit Unicode property tests such as \p{Nd}.
266     Alternatively, if you set the PCRE_UCP option, the way that the character
267     escapes work is changed so that Unicode properties are used to determine which
268     characters match. There are more details in the section on
269     <a href="pcrepattern.html#genericchartypes">generic character types</a>
270     in the
271     <a href="pcrepattern.html"><b>pcrepattern</b></a>
272 ph10 535 documentation.
273 nigel 63 </P>
274     <P>
275 ph10 211 7. Similarly, characters that match the POSIX named character classes are all
276 ph10 518 low-valued characters, unless the PCRE_UCP option is set.
277 nigel 63 </P>
278     <P>
279 ph10 211 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
280 ph10 535 (\h, \H, \v, and \V) do match all the appropriate Unicode characters,
281 ph10 518 whether or not PCRE_UCP is set.
282 ph10 182 </P>
283     <P>
284 ph10 211 9. Case-insensitive matching applies only to characters whose values are less
285 nigel 75 than 128, unless PCRE is built with Unicode property support. Even when Unicode
286     property support is available, PCRE still uses its own character tables when
287     checking the case of low-valued characters, so as not to degrade performance.
288     The Unicode property information is used only for characters with higher
289 nigel 87 values. Even when Unicode property support is available, PCRE supports
290     case-insensitive matching only when there is a one-to-one mapping between a
291     letter's cases. There are a small number of many-to-one mappings in Unicode;
292     these are not supported by PCRE.
293 nigel 63 </P>
294     <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
295     <P>
296 nigel 77 Philip Hazel
297 nigel 63 <br>
298 ph10 99 University Computing Service
299 nigel 63 <br>
300 nigel 93 Cambridge CB2 3QH, England.
301 ph10 99 <br>
302 nigel 77 </P>
303     <P>
304     Putting an actual email address here seems to have been a spam magnet, so I've
305 ph10 153 taken it away. If you want to email me, use my two initials, followed by the
306     two digits 10, at the domain cam.ac.uk.
307 ph10 99 </P>
308     <br><a name="SEC6" href="#TOC1">REVISION</a><br>
309     <P>
310 ph10 518 Last updated: 12 May 2010
311 nigel 63 <br>
312 ph10 507 Copyright &copy; 1997-2010 University of Cambridge.
313 ph10 99 <br>
314 nigel 75 <p>
315     Return to the <a href="index.html">PCRE index page</a>.
316     </p>


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