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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 572 The current implementation of PCRE corresponds approximately with Perl 5.12,
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 5.2.0.
37 nigel 93 </P>
38     <P>
39     In addition to the Perl-compatible matching function, PCRE contains an
40 ph10 461 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 nigel 77 <a href="pcrematching.html"><b>pcrematching</b></a>
44     page.
45     </P>
46     <P>
47 nigel 75 PCRE is written in C and released as a C library. A number of people have
48 nigel 77 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 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>
55 nigel 63 </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 ph10 208 pages. There is a syntax summary in the
63     <a href="pcresyntax.html"><b>pcresyntax</b></a>
64     page.
65 nigel 63 </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 nigel 75 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 ph10 461 found in the <b>README</b> and <b>NON-UNIX-USE</b> files in the source
75     distribution.
76 nigel 63 </P>
77 nigel 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 nigel 83 "_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 nigel 77 </P>
86 nigel 63 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
87     <P>
88 nigel 75 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 ph10 429 all the sections, except the <b>pcredemo</b> section, are concatenated, for ease
92     of searching. The sections are as follows:
93 nigel 63 <pre>
94     pcre this document
95 ph10 153 pcre-config show PCRE installation configuration information
96 nigel 77 pcreapi details of PCRE's native C API
97 nigel 63 pcrebuild options for building PCRE
98     pcrecallout details of the callout feature
99     pcrecompat discussion of Perl compatibility
100 nigel 77 pcrecpp details of the C++ wrapper
101 ph10 429 pcredemo a demonstration C program that uses PCRE
102 nigel 63 pcregrep description of the <b>pcregrep</b> command
103 nigel 77 pcrematching discussion of the two matching algorithms
104 nigel 75 pcrepartial details of the partial matching facility
105     pcrepattern syntax and semantics of supported regular expressions
106 nigel 63 pcreperform discussion of performance issues
107 nigel 77 pcreposix the POSIX-compatible C API
108 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
109 ph10 429 pcresample discussion of the pcredemo program
110 nigel 91 pcrestack discussion of stack usage
111 ph10 461 pcresyntax quick syntax reference
112 nigel 75 pcretest description of the <b>pcretest</b> testing command
113     </pre>
114 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
115 nigel 77 C library function, listing its arguments and results.
116 nigel 63 </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 nigel 75 documentation for details). In these cases the limit is substantially larger.
130 nigel 93 However, the speed of execution is slower.
131 nigel 63 </P>
132     <P>
133 ph10 208 All values in repeating quantifiers must be less than 65536.
134 nigel 63 </P>
135     <P>
136 nigel 93 There is no limit to the number of parenthesized subpatterns, but there can be
137     no more than 65535 capturing subpatterns.
138 nigel 63 </P>
139     <P>
140 nigel 93 The maximum length of name for a named subpattern is 32 characters, and the
141     maximum number of named subpatterns is 10000.
142 nigel 91 </P>
143     <P>
144 nigel 63 The maximum length of a subject string is the largest positive number that an
145 nigel 77 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 nigel 91 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 nigel 75 <a name="utf8support"></a></P>
153     <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
154 nigel 63 <P>
155 nigel 75 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 nigel 63 </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 ph10 416 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 ph10 461 strings of 1-byte characters.
168 nigel 63 </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 nigel 93 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
173 nigel 63 </P>
174     <P>
175 nigel 75 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 nigel 87 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 nigel 75 <a href="pcrepattern.html"><b>pcrepattern</b></a>
182 nigel 87 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 ph10 211 <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 nigel 75 </P>
199     <P>
200 ph10 211 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 nigel 63 </P>
209     <P>
210 ph10 211 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 nigel 63 </P>
218     <P>
219 ph10 211 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 nigel 87 UTF-8 character if the value is greater than 127.
239 nigel 63 </P>
240     <P>
241 ph10 211 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
242 nigel 91 characters for values greater than \177.
243     </P>
244     <P>
245 ph10 211 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
246 nigel 63 bytes, for example: \x{100}{3}.
247     </P>
248     <P>
249 ph10 211 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
250 nigel 63 </P>
251     <P>
252 ph10 211 5. The escape sequence \C can be used to match a single byte in UTF-8 mode,
253 nigel 77 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 nigel 63 </P>
256     <P>
257 ph10 211 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
258 ph10 518 test characters of any code value, but, by default, the characters that PCRE
259     recognizes as digits, spaces, or word characters remain the same set as before,
260     all with values less than 256. This remains true even when PCRE is built to
261     include Unicode property support, because to do otherwise would slow down PCRE
262 ph10 567 in many common cases. Note in particular that this applies to \b and \B,
263     because they are defined in terms of \w and \W. If you really want to test
264     for a wider sense of, say, "digit", you can use explicit Unicode property tests
265     such as \p{Nd}. Alternatively, if you set the PCRE_UCP option, the way that
266     the character escapes work is changed so that Unicode properties are used to
267     determine which characters match. There are more details in the section on
268 ph10 518 <a href="pcrepattern.html#genericchartypes">generic character types</a>
269     in the
270     <a href="pcrepattern.html"><b>pcrepattern</b></a>
271 ph10 535 documentation.
272 nigel 63 </P>
273     <P>
274 ph10 211 7. Similarly, characters that match the POSIX named character classes are all
275 ph10 518 low-valued characters, unless the PCRE_UCP option is set.
276 nigel 63 </P>
277     <P>
278 ph10 572 8. However, the horizontal and vertical whitespace matching escapes (\h, \H,
279     \v, and \V) do match all the appropriate Unicode characters, whether or not
280     PCRE_UCP is set.
281 ph10 182 </P>
282     <P>
283 ph10 211 9. Case-insensitive matching applies only to characters whose values are less
284 nigel 75 than 128, unless PCRE is built with Unicode property support. Even when Unicode
285     property support is available, PCRE still uses its own character tables when
286     checking the case of low-valued characters, so as not to degrade performance.
287     The Unicode property information is used only for characters with higher
288 ph10 572 values. Furthermore, PCRE supports case-insensitive matching only when there is
289     a one-to-one mapping between a letter's cases. There are a small number of
290     many-to-one mappings in Unicode; these are not supported by PCRE.
291 nigel 63 </P>
292     <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
293     <P>
294 nigel 77 Philip Hazel
295 nigel 63 <br>
296 ph10 99 University Computing Service
297 nigel 63 <br>
298 nigel 93 Cambridge CB2 3QH, England.
299 ph10 99 <br>
300 nigel 77 </P>
301     <P>
302     Putting an actual email address here seems to have been a spam magnet, so I've
303 ph10 153 taken it away. If you want to email me, use my two initials, followed by the
304     two digits 10, at the domain cam.ac.uk.
305 ph10 99 </P>
306     <br><a name="SEC6" href="#TOC1">REVISION</a><br>
307     <P>
308 ph10 572 Last updated: 13 November 2010
309 nigel 63 <br>
310 ph10 507 Copyright &copy; 1997-2010 University of Cambridge.
311 ph10 99 <br>
312 nigel 75 <p>
313     Return to the <a href="index.html">PCRE index page</a>.
314     </p>


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