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Revision 518 - (hide annotations) (download)
Tue May 18 15:47:01 2010 UTC (3 years, 11 months ago) by ph10
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Added PCRE_UCP and related stuff to make \w etc use Unicode properties.

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 456 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 nigel 75 .P
14 ph10 518 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 nigel 93 .P
20     In addition to the Perl-compatible matching function, PCRE contains an
21 ph10 456 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 nigel 77 .\" 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 ph10 456 found in the \fBREADME\fP and \fBNON-UNIX-USE\fP files in the source
70     distribution.
71 nigel 77 .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 nigel 83 "_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 nigel 75 .
80     .
81     .SH "USER DOCUMENTATION"
82 nigel 63 .rs
83     .sp
84 nigel 75 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 ph10 429 all the sections, except the \fBpcredemo\fP section, are concatenated, for ease
88     of searching. The sections are as follows:
89 nigel 75 .sp
90 nigel 63 pcre this document
91 ph10 153 pcre-config show PCRE installation configuration information
92 nigel 77 pcreapi details of PCRE's native C API
93 nigel 63 pcrebuild options for building PCRE
94     pcrecallout details of the callout feature
95     pcrecompat discussion of Perl compatibility
96 nigel 77 pcrecpp details of the C++ wrapper
97 ph10 429 pcredemo a demonstration C program that uses PCRE
98 nigel 75 pcregrep description of the \fBpcregrep\fP command
99 nigel 77 pcrematching discussion of the two matching algorithms
100 nigel 75 pcrepartial details of the partial matching facility
101     .\" JOIN
102 nigel 63 pcrepattern syntax and semantics of supported
103     regular expressions
104     pcreperform discussion of performance issues
105 nigel 77 pcreposix the POSIX-compatible C API
106 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
107 ph10 429 pcresample discussion of the pcredemo program
108 nigel 91 pcrestack discussion of stack usage
109 ph10 456 pcresyntax quick syntax reference
110 nigel 75 pcretest description of the \fBpcretest\fP testing command
111     .sp
112 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
113 nigel 77 C library function, listing its arguments and results.
114 nigel 75 .
115     .
116 nigel 41 .SH LIMITATIONS
117 nigel 63 .rs
118     .sp
119 nigel 41 There are some size limitations in PCRE but it is hoped that they will never in
120     practice be relevant.
121 nigel 75 .P
122 nigel 63 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 nigel 75 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
126 nigel 63 distribution and the
127     .\" HREF
128 nigel 75 \fBpcrebuild\fP
129 nigel 63 .\"
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 75 .P
133 ph10 208 All values in repeating quantifiers must be less than 65536.
134 nigel 75 .P
135 nigel 93 There is no limit to the number of parenthesized subpatterns, but there can be
136     no more than 65535 capturing subpatterns.
137 nigel 75 .P
138 nigel 93 The maximum length of name for a named subpattern is 32 characters, and the
139     maximum number of named subpatterns is 10000.
140 nigel 91 .P
141 nigel 41 The maximum length of a subject string is the largest positive number that an
142 nigel 77 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 nigel 91 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 ph10 211 .
152 ph10 456 .
153 nigel 63 .\" HTML <a name="utf8support"></a>
154 nigel 75 .SH "UTF-8 AND UNICODE PROPERTY SUPPORT"
155 nigel 63 .rs
156     .sp
157 nigel 75 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 nigel 63 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 nigel 75 \fBpcre_compile()\fP
166 nigel 63 .\"
167 ph10 412 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 ph10 456 strings of 1-byte characters.
171 nigel 75 .P
172 nigel 49 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 nigel 93 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
175 nigel 75 .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 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 .\" HREF
183     \fBpcrepattern\fP
184     .\"
185 nigel 87 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 ph10 211 .
190 ph10 456 .
191 ph10 211 .\" 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 nigel 75 .P
203 ph10 211 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 nigel 75 .P
212 ph10 211 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 nigel 75 .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 ph10 209 .P
228 ph10 211 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 ph10 518 .
234 ph10 211 .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 nigel 87 UTF-8 character if the value is greater than 127.
239 nigel 75 .P
240 ph10 211 2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
241 nigel 91 characters for values greater than \e177.
242     .P
243 ph10 211 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
244 nigel 75 bytes, for example: \ex{100}{3}.
245     .P
246 ph10 211 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
247 nigel 75 .P
248 ph10 211 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
249 nigel 77 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 nigel 75 .P
252 ph10 211 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
253 ph10 518 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 nigel 75 .P
273 ph10 211 7. Similarly, characters that match the POSIX named character classes are all
274 ph10 518 low-valued characters, unless the PCRE_UCP option is set.
275 nigel 75 .P
276 ph10 211 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
277 ph10 518 (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters,
278     whether or not PCRE_UCP is set.
279 ph10 181 .P
280 ph10 211 9. Case-insensitive matching applies only to characters whose values are less
281 nigel 75 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 nigel 87 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 nigel 75 .
290 ph10 99 .
291 nigel 41 .SH AUTHOR
292 nigel 63 .rs
293     .sp
294 ph10 99 .nf
295 nigel 77 Philip Hazel
296 ph10 99 University Computing Service
297 nigel 93 Cambridge CB2 3QH, England.
298 ph10 99 .fi
299 nigel 77 .P
300     Putting an actual email address here seems to have been a spam magnet, so I've
301 ph10 153 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 ph10 99 .
304     .
305     .SH REVISION
306     .rs
307 nigel 75 .sp
308 ph10 99 .nf
309 ph10 518 Last updated: 12 May 2010
310 ph10 491 Copyright (c) 1997-2010 University of Cambridge.
311 ph10 99 .fi

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