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Wed Mar 18 16:38:23 2009 UTC (12 years, 6 months ago) by ph10
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Add words about \b in UTF-8 mode.
1 .TH PCRE 3
3 PCRE - Perl-compatible regular expressions
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. Certain features that appeared in Python and PCRE before they
10 appeared in Perl are also available using the Python syntax. There is also some
11 support for certain .NET and Oniguruma syntax items, and there is an option for
12 requesting some minor changes that give better JavaScript compatibility.
13 .P
14 The current implementation of PCRE (release 7.x) corresponds approximately with
15 Perl 5.10, 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 5.0.0.
19 .P
20 In addition to the Perl-compatible matching function, PCRE contains an
21 alternative matching function that matches the same compiled patterns in a
22 different way. In certain circumstances, the alternative function has some
23 advantages. 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 file in the source distribution.
70 .P
71 The library contains a number of undocumented internal functions and data
72 tables that are used by more than one of the exported external functions, but
73 which are not intended for use by external callers. Their names all begin with
74 "_pcre_", which hopefully will not provoke any name clashes. In some
75 environments, it is possible to control which external symbols are exported
76 when a shared library is built, and in these cases the undocumented symbols are
77 not exported.
78 .
79 .
81 .rs
82 .sp
83 The user documentation for PCRE comprises a number of different sections. In
84 the "man" format, each of these is a separate "man page". In the HTML format,
85 each is a separate page, linked from the index page. In the plain text format,
86 all the sections are concatenated, for ease of searching. The sections are as
87 follows:
88 .sp
89 pcre this document
90 pcre-config show PCRE installation configuration information
91 pcreapi details of PCRE's native C API
92 pcrebuild options for building PCRE
93 pcrecallout details of the callout feature
94 pcrecompat discussion of Perl compatibility
95 pcrecpp details of the C++ wrapper
96 pcregrep description of the \fBpcregrep\fP command
97 pcrematching discussion of the two matching algorithms
98 pcrepartial details of the partial matching facility
99 .\" JOIN
100 pcrepattern syntax and semantics of supported
101 regular expressions
102 pcresyntax quick syntax reference
103 pcreperform discussion of performance issues
104 pcreposix the POSIX-compatible C API
105 pcreprecompile details of saving and re-using precompiled patterns
106 pcresample discussion of the sample program
107 pcrestack discussion of stack usage
108 pcretest description of the \fBpcretest\fP testing command
109 .sp
110 In addition, in the "man" and HTML formats, there is a short page for each
111 C library function, listing its arguments and results.
112 .
113 .
115 .rs
116 .sp
117 There are some size limitations in PCRE but it is hoped that they will never in
118 practice be relevant.
119 .P
120 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
121 compiled with the default internal linkage size of 2. If you want to process
122 regular expressions that are truly enormous, you can compile PCRE with an
123 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
124 distribution and the
125 .\" HREF
126 \fBpcrebuild\fP
127 .\"
128 documentation for details). In these cases the limit is substantially larger.
129 However, the speed of execution is slower.
130 .P
131 All values in repeating quantifiers must be less than 65536.
132 .P
133 There is no limit to the number of parenthesized subpatterns, but there can be
134 no more than 65535 capturing subpatterns.
135 .P
136 The maximum length of name for a named subpattern is 32 characters, and the
137 maximum number of named subpatterns is 10000.
138 .P
139 The maximum length of a subject string is the largest positive number that an
140 integer variable can hold. However, when using the traditional matching
141 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
142 This means that the available stack space may limit the size of a subject
143 string that can be processed by certain patterns. For a discussion of stack
144 issues, see the
145 .\" HREF
146 \fBpcrestack\fP
147 .\"
148 documentation.
149 .
150 .\" HTML <a name="utf8support"></a>
151 .
152 .
154 .rs
155 .sp
156 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 .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 .\" HREF
164 \fBpcre_compile()\fP
165 .\"
166 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
167 subject strings that are matched against it are treated as UTF-8 strings
168 instead of just strings of bytes.
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 If PCRE is built with Unicode character property support (which implies UTF-8
175 support), the escape sequences \ep{..}, \eP{..}, and \eX 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 .\" HREF
181 \fBpcrepattern\fP
182 .\"
183 documentation. Only the short names for properties are supported. For example,
184 \ep{L} matches a letter. Its Perl synonym, \ep{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 .
188 .\" HTML <a name="utf8strings"></a>
189 .
190 .SS "Validity of UTF-8 strings"
191 .rs
192 .sp
193 When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
194 are (by default) checked for validity on entry to the relevant functions. From
195 release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
196 themselves derived from the Unicode specification. Earlier releases of PCRE
197 followed the rules of RFC 2279, which allows the full range of 31-bit values (0
198 to 0x7FFFFFFF). The current check allows only values in the range U+0 to
199 U+10FFFF, excluding U+D800 to U+DFFF.
200 .P
201 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 .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 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 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
227 encoded in a UTF-8-like manner as per the old RFC, you can set
228 PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
229 situation, you will have to apply your own validity check.
230 .
231 .SS "General comments about UTF-8 mode"
232 .rs
233 .sp
234 1. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
235 UTF-8 character if the value is greater than 127.
236 .P
237 2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
238 characters for values greater than \e177.
239 .P
240 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
241 bytes, for example: \ex{100}{3}.
242 .P
243 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
244 .P
245 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
246 but its use can lead to some strange effects. This facility is not available in
247 the alternative matching function, \fBpcre_dfa_exec()\fP.
248 .P
249 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
250 test characters of any code value, but the characters that PCRE recognizes as
251 digits, spaces, or word characters remain the same set as before, all with
252 values less than 256. This remains true even when PCRE includes Unicode
253 property support, because to do otherwise would slow down PCRE in many common
254 cases. If you really want to test for a wider sense of, say, "digit", you
255 must use Unicode property tests such as \ep{Nd}. Note that this also applies to
256 \eb, because it is defined in terms of \ew and \eW.
257 .P
258 7. Similarly, characters that match the POSIX named character classes are all
259 low-valued characters.
260 .P
261 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
262 (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
263 .P
264 9. Case-insensitive matching applies only to characters whose values are less
265 than 128, unless PCRE is built with Unicode property support. Even when Unicode
266 property support is available, PCRE still uses its own character tables when
267 checking the case of low-valued characters, so as not to degrade performance.
268 The Unicode property information is used only for characters with higher
269 values. Even when Unicode property support is available, PCRE supports
270 case-insensitive matching only when there is a one-to-one mapping between a
271 letter's cases. There are a small number of many-to-one mappings in Unicode;
272 these are not supported by PCRE.
273 .
274 .
276 .rs
277 .sp
278 .nf
279 Philip Hazel
280 University Computing Service
281 Cambridge CB2 3QH, England.
282 .fi
283 .P
284 Putting an actual email address here seems to have been a spam magnet, so I've
285 taken it away. If you want to email me, use my two initials, followed by the
286 two digits 10, at the domain cam.ac.uk.
287 .
288 .
290 .rs
291 .sp
292 .nf
293 Last updated: 18 March 2009
294 Copyright (c) 1997-2009 University of Cambridge.
295 .fi


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