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1 nigel 41 .TH PCRE 3
2     .SH NAME
3 nigel 63 PCRE - Perl-compatible regular expressions
4 nigel 75 .SH INTRODUCTION
5 nigel 63 .rs
6     .sp
7 nigel 41 The PCRE library is a set of functions that implement regular expression
8 nigel 63 pattern matching using the same syntax and semantics as Perl, with just a few
9 ph10 336 differences. Certain features that appeared in Python and PCRE before they
10 ph10 345 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 ph10 336 requesting some minor changes that give better JavaScript compatibility.
13 nigel 75 .P
14 nigel 93 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 nigel 77 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 nigel 75 PCRE is written in C and released as a C library. A number of people have
30 nigel 77 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 nigel 75 .sp
39 nigel 63 .\" 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 nigel 75 .P
43 nigel 63 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 ph10 208 pages. There is a syntax summary in the
53     .\" HREF
54     \fBpcresyntax\fR
55     .\"
56     page.
57 nigel 75 .P
58 nigel 63 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 nigel 75 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 nigel 77 .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 nigel 83 "_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 nigel 75 .
79     .
80     .SH "USER DOCUMENTATION"
81 nigel 63 .rs
82     .sp
83 nigel 75 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 nigel 63 pcre this document
90 ph10 153 pcre-config show PCRE installation configuration information
91 nigel 77 pcreapi details of PCRE's native C API
92 nigel 63 pcrebuild options for building PCRE
93     pcrecallout details of the callout feature
94     pcrecompat discussion of Perl compatibility
95 nigel 77 pcrecpp details of the C++ wrapper
96 nigel 75 pcregrep description of the \fBpcregrep\fP command
97 nigel 77 pcrematching discussion of the two matching algorithms
98 nigel 75 pcrepartial details of the partial matching facility
99     .\" JOIN
100 nigel 63 pcrepattern syntax and semantics of supported
101     regular expressions
102 ph10 208 pcresyntax quick syntax reference
103 nigel 63 pcreperform discussion of performance issues
104 nigel 77 pcreposix the POSIX-compatible C API
105 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
106 nigel 63 pcresample discussion of the sample program
107 nigel 91 pcrestack discussion of stack usage
108 nigel 75 pcretest description of the \fBpcretest\fP testing command
109     .sp
110 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
111 nigel 77 C library function, listing its arguments and results.
112 nigel 75 .
113     .
114 nigel 41 .SH LIMITATIONS
115 nigel 63 .rs
116     .sp
117 nigel 41 There are some size limitations in PCRE but it is hoped that they will never in
118     practice be relevant.
119 nigel 75 .P
120 nigel 63 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 nigel 75 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
124 nigel 63 distribution and the
125     .\" HREF
126 nigel 75 \fBpcrebuild\fP
127 nigel 63 .\"
128 nigel 75 documentation for details). In these cases the limit is substantially larger.
129 nigel 93 However, the speed of execution is slower.
130 nigel 75 .P
131 ph10 208 All values in repeating quantifiers must be less than 65536.
132 nigel 75 .P
133 nigel 93 There is no limit to the number of parenthesized subpatterns, but there can be
134     no more than 65535 capturing subpatterns.
135 nigel 75 .P
136 nigel 93 The maximum length of name for a named subpattern is 32 characters, and the
137     maximum number of named subpatterns is 10000.
138 nigel 91 .P
139 nigel 41 The maximum length of a subject string is the largest positive number that an
140 nigel 77 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 nigel 91 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 ph10 211 .
150 nigel 63 .\" HTML <a name="utf8support"></a>
151 nigel 75 .
152     .
153     .SH "UTF-8 AND UNICODE PROPERTY SUPPORT"
154 nigel 63 .rs
155     .sp
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     .P
161 nigel 63 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 nigel 75 \fBpcre_compile()\fP
165 nigel 63 .\"
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 nigel 75 .P
170 nigel 49 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 75 .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 nigel 87 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 nigel 75 .\" HREF
181     \fBpcrepattern\fP
182     .\"
183 nigel 87 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 ph10 211 .
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 nigel 75 .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 75 .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 75 .P
218 ph10 211 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 ph10 209 .P
226 ph10 211 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 nigel 87 UTF-8 character if the value is greater than 127.
236 nigel 75 .P
237 ph10 211 2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
238 nigel 91 characters for values greater than \e177.
239     .P
240 ph10 211 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
241 nigel 75 bytes, for example: \ex{100}{3}.
242     .P
243 ph10 211 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
244 nigel 75 .P
245 ph10 211 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
246 nigel 77 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 nigel 75 .P
249 ph10 211 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
250 nigel 63 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 nigel 75 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}.
256     .P
257 ph10 211 7. Similarly, characters that match the POSIX named character classes are all
258 nigel 75 low-valued characters.
259     .P
260 ph10 211 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
261 ph10 181 (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
262     .P
263 ph10 211 9. Case-insensitive matching applies only to characters whose values are less
264 nigel 75 than 128, unless PCRE is built with Unicode property support. Even when Unicode
265     property support is available, PCRE still uses its own character tables when
266     checking the case of low-valued characters, so as not to degrade performance.
267     The Unicode property information is used only for characters with higher
268 nigel 87 values. Even when Unicode property support is available, PCRE supports
269     case-insensitive matching only when there is a one-to-one mapping between a
270     letter's cases. There are a small number of many-to-one mappings in Unicode;
271     these are not supported by PCRE.
272 nigel 75 .
273 ph10 99 .
274 nigel 41 .SH AUTHOR
275 nigel 63 .rs
276     .sp
277 ph10 99 .nf
278 nigel 77 Philip Hazel
279 ph10 99 University Computing Service
280 nigel 93 Cambridge CB2 3QH, England.
281 ph10 99 .fi
282 nigel 77 .P
283     Putting an actual email address here seems to have been a spam magnet, so I've
284 ph10 153 taken it away. If you want to email me, use my two initials, followed by the
285     two digits 10, at the domain cam.ac.uk.
286 ph10 99 .
287     .
288     .SH REVISION
289     .rs
290 nigel 75 .sp
291 ph10 99 .nf
292 ph10 336 Last updated: 12 April 2008
293     Copyright (c) 1997-2008 University of Cambridge.
294 ph10 99 .fi

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