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1 .TH PCRE 3
2 .SH NAME
3 PCRE - Perl-compatible regular expressions
4 .SH INTRODUCTION
5 .rs
6 .sp
7 The PCRE library is a set of functions that implement regular expression
8 pattern matching using the same syntax and semantics as Perl, with just a few
9 differences. Some features that appeared in Python and PCRE before they
10 appeared in Perl are also available using the Python syntax, there is some
11 support for one or two .NET and Oniguruma syntax items, and there is an option
12 for requesting some minor changes that give better JavaScript compatibility.
13 .P
14 The current implementation of PCRE corresponds approximately with Perl
15 5.10/5.11, including support for UTF-8 encoded strings and Unicode general
16 category properties. However, UTF-8 and Unicode support has to be explicitly
17 enabled; it is not the default. The Unicode tables correspond to Unicode
18 release 5.2.0.
19 .P
20 In addition to the Perl-compatible matching function, PCRE contains an
21 alternative function that matches the same compiled patterns in a different
22 way. In certain circumstances, the alternative function has some advantages.
23 For a discussion of the two matching algorithms, see the
24 .\" HREF
25 \fBpcrematching\fP
26 .\"
27 page.
28 .P
29 PCRE is written in C and released as a C library. A number of people have
30 written wrappers and interfaces of various kinds. In particular, Google Inc.
31 have provided a comprehensive C++ wrapper. This is now included as part of the
32 PCRE distribution. The
33 .\" HREF
34 \fBpcrecpp\fP
35 .\"
36 page has details of this interface. Other people's contributions can be found
37 in the \fIContrib\fR directory at the primary FTP site, which is:
38 .sp
39 .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
40 .\" </a>
41 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
42 .P
43 Details of exactly which Perl regular expression features are and are not
44 supported by PCRE are given in separate documents. See the
45 .\" HREF
46 \fBpcrepattern\fR
47 .\"
48 and
49 .\" HREF
50 \fBpcrecompat\fR
51 .\"
52 pages. There is a syntax summary in the
53 .\" HREF
54 \fBpcresyntax\fR
55 .\"
56 page.
57 .P
58 Some features of PCRE can be included, excluded, or changed when the library is
59 built. The
60 .\" HREF
61 \fBpcre_config()\fR
62 .\"
63 function makes it possible for a client to discover which features are
64 available. The features themselves are described in the
65 .\" HREF
66 \fBpcrebuild\fP
67 .\"
68 page. Documentation about building PCRE for various operating systems can be
69 found in the \fBREADME\fP and \fBNON-UNIX-USE\fP files in the source
70 distribution.
71 .P
72 The library contains a number of undocumented internal functions and data
73 tables that are used by more than one of the exported external functions, but
74 which are not intended for use by external callers. Their names all begin with
75 "_pcre_", which hopefully will not provoke any name clashes. In some
76 environments, it is possible to control which external symbols are exported
77 when a shared library is built, and in these cases the undocumented symbols are
78 not exported.
79 .
80 .
81 .SH "USER DOCUMENTATION"
82 .rs
83 .sp
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, except the \fBpcredemo\fP section, are concatenated, for ease
88 of searching. The sections are as follows:
89 .sp
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 pcredemo a demonstration C program that uses PCRE
98 pcregrep description of the \fBpcregrep\fP command
99 pcrematching discussion of the two matching algorithms
100 pcrepartial details of the partial matching facility
101 .\" JOIN
102 pcrepattern syntax and semantics of supported
103 regular expressions
104 pcreperform discussion of performance issues
105 pcreposix the POSIX-compatible C API
106 pcreprecompile details of saving and re-using precompiled patterns
107 pcresample discussion of the pcredemo program
108 pcrestack discussion of stack usage
109 pcresyntax quick syntax reference
110 pcretest description of the \fBpcretest\fP testing command
111 .sp
112 In addition, in the "man" and HTML formats, there is a short page for each
113 C library function, listing its arguments and results.
114 .
115 .
116 .SH LIMITATIONS
117 .rs
118 .sp
119 There are some size limitations in PCRE but it is hoped that they will never in
120 practice be relevant.
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 \fBREADME\fP file in the source
126 distribution and the
127 .\" HREF
128 \fBpcrebuild\fP
129 .\"
130 documentation for details). In these cases the limit is substantially larger.
131 However, the speed of execution is slower.
132 .P
133 All values in repeating quantifiers must be less than 65536.
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 The maximum length of name for a named subpattern is 32 characters, and the
139 maximum number of named subpatterns is 10000.
140 .P
141 The maximum length of a subject string is the largest positive number that an
142 integer variable can hold. However, when using the traditional matching
143 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
144 This means that the available stack space may limit the size of a subject
145 string that can be processed by certain patterns. For a discussion of stack
146 issues, see the
147 .\" HREF
148 \fBpcrestack\fP
149 .\"
150 documentation.
151 .
152 .
153 .\" HTML <a name="utf8support"></a>
154 .SH "UTF-8 AND UNICODE PROPERTY SUPPORT"
155 .rs
156 .sp
157 From release 3.3, PCRE has had some support for character strings encoded in
158 the UTF-8 format. For release 4.0 this was greatly extended to cover most
159 common requirements, and in release 5.0 additional support for Unicode general
160 category properties was added.
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 .\" HREF
165 \fBpcre_compile()\fP
166 .\"
167 with the PCRE_UTF8 option flag, or the pattern must start with the sequence
168 (*UTF8). When either of these is the case, both the pattern and any subject
169 strings that are matched against it are treated as UTF-8 strings instead of
170 strings of 1-byte characters.
171 .P
172 If you compile PCRE with UTF-8 support, but do not use it at run time, the
173 library will be a bit bigger, but the additional run time overhead is limited
174 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
175 .P
176 If PCRE is built with Unicode character property support (which implies UTF-8
177 support), the escape sequences \ep{..}, \eP{..}, and \eX 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 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 .\" HREF
183 \fBpcrepattern\fP
184 .\"
185 documentation. Only the short names for properties are supported. For example,
186 \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
187 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
188 compatibility with Perl 5.6. PCRE does not support this.
189 .
190 .
191 .\" HTML <a name="utf8strings"></a>
192 .SS "Validity of UTF-8 strings"
193 .rs
194 .sp
195 When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
196 are (by default) checked for validity on entry to the relevant functions. From
197 release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
198 themselves derived from the Unicode specification. Earlier releases of PCRE
199 followed the rules of RFC 2279, which allows the full range of 31-bit values (0
200 to 0x7FFFFFFF). The current check allows only values in the range U+0 to
201 U+10FFFF, excluding U+D800 to U+DFFF.
202 .P
203 The excluded code points are the "Low Surrogate Area" of Unicode, of which the
204 Unicode Standard says this: "The Low Surrogate Area does not contain any
205 character assignments, consequently no character code charts or namelists are
206 provided for this area. Surrogates are reserved for use with UTF-16 and then
207 must be used in pairs." The code points that are encoded by UTF-16 pairs are
208 available as independent code points in the UTF-8 encoding. (In other words,
209 the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
210 UTF-8.)
211 .P
212 If an invalid UTF-8 string is passed to PCRE, an error return
213 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
214 your strings are valid, and therefore want to skip these checks in order to
215 improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
216 at run time, PCRE assumes that the pattern or subject it is given
217 (respectively) contains only valid UTF-8 codes. In this case, it does not
218 diagnose an invalid UTF-8 string.
219 .P
220 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 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 .
233 .
234 .SS "General comments about UTF-8 mode"
235 .rs
236 .sp
237 1. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
238 UTF-8 character if the value is greater than 127.
239 .P
240 2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
241 characters for values greater than \e177.
242 .P
243 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
244 bytes, for example: \ex{100}{3}.
245 .P
246 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
247 .P
248 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
249 but its use can lead to some strange effects. This facility is not available in
250 the alternative matching function, \fBpcre_dfa_exec()\fP.
251 .P
252 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
253 test characters of any code value, but, by default, the characters that PCRE
254 recognizes as digits, spaces, or word characters remain the same set as before,
255 all with values less than 256. This remains true even when PCRE is built to
256 include Unicode property support, because to do otherwise would slow down PCRE
257 in many common cases. Note that this also applies to \eb, because it is defined
258 in terms of \ew and \eW. If you really want to test for a wider sense of, say,
259 "digit", you can use explicit Unicode property tests such as \ep{Nd}.
260 Alternatively, if you set the PCRE_UCP option, the way that the character
261 escapes work is changed so that Unicode properties are used to determine which
262 characters match. There are more details in the section on
263 .\" HTML <a href="pcrepattern.html#genericchartypes">
264 .\" </a>
265 generic character types
266 .\"
267 in the
268 .\" HREF
269 \fBpcrepattern\fP
270 .\"
271 documentation.
272 .P
273 7. Similarly, characters that match the POSIX named character classes are all
274 low-valued characters, unless the PCRE_UCP option is set.
275 .P
276 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
277 (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters,
278 whether or not PCRE_UCP is set.
279 .P
280 9. Case-insensitive matching applies only to characters whose values are less
281 than 128, unless PCRE is built with Unicode property support. Even when Unicode
282 property support is available, PCRE still uses its own character tables when
283 checking the case of low-valued characters, so as not to degrade performance.
284 The Unicode property information is used only for characters with higher
285 values. Even when Unicode property support is available, PCRE supports
286 case-insensitive matching only when there is a one-to-one mapping between a
287 letter's cases. There are a small number of many-to-one mappings in Unicode;
288 these are not supported by PCRE.
289 .
290 .
291 .SH AUTHOR
292 .rs
293 .sp
294 .nf
295 Philip Hazel
296 University Computing Service
297 Cambridge CB2 3QH, England.
298 .fi
299 .P
300 Putting an actual email address here seems to have been a spam magnet, so I've
301 taken it away. If you want to email me, use my two initials, followed by the
302 two digits 10, at the domain cam.ac.uk.
303 .
304 .
305 .SH REVISION
306 .rs
307 .sp
308 .nf
309 Last updated: 12 May 2010
310 Copyright (c) 1997-2010 University of Cambridge.
311 .fi

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