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Tue Aug 7 09:22:06 2007 UTC (14 years, 2 months ago) by ph10
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Add words about valid UTF-8 vs assigned Unicode code point to the docs.
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.)
11 .P
12 The current implementation of PCRE (release 7.x) corresponds approximately with
13 Perl 5.10, including support for UTF-8 encoded strings and Unicode general
14 category properties. However, UTF-8 and Unicode support has to be explicitly
15 enabled; it is not the default. The Unicode tables correspond to Unicode
16 release 5.0.0.
17 .P
18 In addition to the Perl-compatible matching function, PCRE contains an
19 alternative matching function that matches the same compiled patterns in a
20 different way. In certain circumstances, the alternative function has some
21 advantages. For a discussion of the two matching algorithms, see the
22 .\" HREF
23 \fBpcrematching\fP
24 .\"
25 page.
26 .P
27 PCRE is written in C and released as a C library. A number of people have
28 written wrappers and interfaces of various kinds. In particular, Google Inc.
29 have provided a comprehensive C++ wrapper. This is now included as part of the
30 PCRE distribution. The
31 .\" HREF
32 \fBpcrecpp\fP
33 .\"
34 page has details of this interface. Other people's contributions can be found
35 in the \fIContrib\fR directory at the primary FTP site, which is:
36 .sp
37 .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
38 .\" </a>
39 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
40 .P
41 Details of exactly which Perl regular expression features are and are not
42 supported by PCRE are given in separate documents. See the
43 .\" HREF
44 \fBpcrepattern\fR
45 .\"
46 and
47 .\" HREF
48 \fBpcrecompat\fR
49 .\"
50 pages. There is a syntax summary in the
51 .\" HREF
52 \fBpcresyntax\fR
53 .\"
54 page.
55 .P
56 Some features of PCRE can be included, excluded, or changed when the library is
57 built. The
58 .\" HREF
59 \fBpcre_config()\fR
60 .\"
61 function makes it possible for a client to discover which features are
62 available. The features themselves are described in the
63 .\" HREF
64 \fBpcrebuild\fP
65 .\"
66 page. Documentation about building PCRE for various operating systems can be
67 found in the \fBREADME\fP file in the source distribution.
68 .P
69 The library contains a number of undocumented internal functions and data
70 tables that are used by more than one of the exported external functions, but
71 which are not intended for use by external callers. Their names all begin with
72 "_pcre_", which hopefully will not provoke any name clashes. In some
73 environments, it is possible to control which external symbols are exported
74 when a shared library is built, and in these cases the undocumented symbols are
75 not exported.
76 .
77 .
79 .rs
80 .sp
81 The user documentation for PCRE comprises a number of different sections. In
82 the "man" format, each of these is a separate "man page". In the HTML format,
83 each is a separate page, linked from the index page. In the plain text format,
84 all the sections are concatenated, for ease of searching. The sections are as
85 follows:
86 .sp
87 pcre this document
88 pcre-config show PCRE installation configuration information
89 pcreapi details of PCRE's native C API
90 pcrebuild options for building PCRE
91 pcrecallout details of the callout feature
92 pcrecompat discussion of Perl compatibility
93 pcrecpp details of the C++ wrapper
94 pcregrep description of the \fBpcregrep\fP command
95 pcrematching discussion of the two matching algorithms
96 pcrepartial details of the partial matching facility
97 .\" JOIN
98 pcrepattern syntax and semantics of supported
99 regular expressions
100 pcresyntax quick syntax reference
101 pcreperform discussion of performance issues
102 pcreposix the POSIX-compatible C API
103 pcreprecompile details of saving and re-using precompiled patterns
104 pcresample discussion of the sample program
105 pcrestack discussion of stack usage
106 pcretest description of the \fBpcretest\fP testing command
107 .sp
108 In addition, in the "man" and HTML formats, there is a short page for each
109 C library function, listing its arguments and results.
110 .
111 .
113 .rs
114 .sp
115 There are some size limitations in PCRE but it is hoped that they will never in
116 practice be relevant.
117 .P
118 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
119 compiled with the default internal linkage size of 2. If you want to process
120 regular expressions that are truly enormous, you can compile PCRE with an
121 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
122 distribution and the
123 .\" HREF
124 \fBpcrebuild\fP
125 .\"
126 documentation for details). In these cases the limit is substantially larger.
127 However, the speed of execution is slower.
128 .P
129 All values in repeating quantifiers must be less than 65536.
130 .P
131 There is no limit to the number of parenthesized subpatterns, but there can be
132 no more than 65535 capturing subpatterns.
133 .P
134 The maximum length of name for a named subpattern is 32 characters, and the
135 maximum number of named subpatterns is 10000.
136 .P
137 The maximum length of a subject string is the largest positive number that an
138 integer variable can hold. However, when using the traditional matching
139 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
140 This means that the available stack space may limit the size of a subject
141 string that can be processed by certain patterns. For a discussion of stack
142 issues, see the
143 .\" HREF
144 \fBpcrestack\fP
145 .\"
146 documentation.
147 .sp
148 .\" HTML <a name="utf8support"></a>
149 .
150 .
152 .rs
153 .sp
154 From release 3.3, PCRE has had some support for character strings encoded in
155 the UTF-8 format. For release 4.0 this was greatly extended to cover most
156 common requirements, and in release 5.0 additional support for Unicode general
157 category properties was added.
158 .P
159 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
160 the code, and, in addition, you must call
161 .\" HREF
162 \fBpcre_compile()\fP
163 .\"
164 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
165 subject strings that are matched against it are treated as UTF-8 strings
166 instead of just strings of bytes.
167 .P
168 If you compile PCRE with UTF-8 support, but do not use it at run time, the
169 library will be a bit bigger, but the additional run time overhead is limited
170 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
171 .P
172 If PCRE is built with Unicode character property support (which implies UTF-8
173 support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
174 The available properties that can be tested are limited to the general
175 category properties such as Lu for an upper case letter or Nd for a decimal
176 number, the Unicode script names such as Arabic or Han, and the derived
177 properties Any and L&. A full list is given in the
178 .\" HREF
179 \fBpcrepattern\fP
180 .\"
181 documentation. Only the short names for properties are supported. For example,
182 \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
183 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
184 compatibility with Perl 5.6. PCRE does not support this.
185 .P
186 The following comments apply when PCRE is running in UTF-8 mode:
187 .P
188 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
189 are checked for validity on entry to the relevant functions. Note that the
190 check is for a syntactically valid UTF-8 byte string, as defined by RFC 2279.
191 It is \fInot\fP a check for a UTF-8 string of assigned or allowable Unicode
192 code points. For example, the byte sequence \exED\exB2\ex94 is a valid UTF-8
193 encoding of the code point U+DC94, and is not rejected by PCRE. However, that
194 code point is in the "Low Surrogate Area" of Unicode, of which the Unicode
195 Standard says this: "The Low Surrogate Area does not contain any character
196 assignments, consequently no character code charts or namelists are provided
197 for this area. Surrogates are reserved for use with UTF-16 and then must be
198 used in pairs."
199 .P
200 The reason for the UTF-8 check at the start is so that the rest of PCRE can
201 assume that UTF-8 strings are well formed. There is no intention of
202 interpreting the values of the code points, which would involve more processing
203 and affect performance.
204 .P
205 If a syntactically invalid UTF-8 string is passed, an error return is given. In
206 some situations, you may already know that your strings are valid, and
207 therefore want to skip these checks in order to improve performance. If you set
208 the PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes that
209 the pattern or subject it is given (respectively) contains only valid UTF-8
210 codes. In this case, it does not diagnose an invalid UTF-8 string. If you pass
211 an invalid UTF-8 string to PCRE when PCRE_NO_UTF8_CHECK is set, the results are
212 undefined. Your program may crash.
213 .P
214 2. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
215 UTF-8 character if the value is greater than 127.
216 .P
217 3. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
218 characters for values greater than \e177.
219 .P
220 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
221 bytes, for example: \ex{100}{3}.
222 .P
223 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
224 .P
225 6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
226 but its use can lead to some strange effects. This facility is not available in
227 the alternative matching function, \fBpcre_dfa_exec()\fP.
228 .P
229 7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
230 test characters of any code value, but the characters that PCRE recognizes as
231 digits, spaces, or word characters remain the same set as before, all with
232 values less than 256. This remains true even when PCRE includes Unicode
233 property support, because to do otherwise would slow down PCRE in many common
234 cases. If you really want to test for a wider sense of, say, "digit", you
235 must use Unicode property tests such as \ep{Nd}.
236 .P
237 8. Similarly, characters that match the POSIX named character classes are all
238 low-valued characters.
239 .P
240 9. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
241 (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
242 .P
243 10. Case-insensitive matching applies only to characters whose values are less
244 than 128, unless PCRE is built with Unicode property support. Even when Unicode
245 property support is available, PCRE still uses its own character tables when
246 checking the case of low-valued characters, so as not to degrade performance.
247 The Unicode property information is used only for characters with higher
248 values. Even when Unicode property support is available, PCRE supports
249 case-insensitive matching only when there is a one-to-one mapping between a
250 letter's cases. There are a small number of many-to-one mappings in Unicode;
251 these are not supported by PCRE.
252 .
253 .
255 .rs
256 .sp
257 .nf
258 Philip Hazel
259 University Computing Service
260 Cambridge CB2 3QH, England.
261 .fi
262 .P
263 Putting an actual email address here seems to have been a spam magnet, so I've
264 taken it away. If you want to email me, use my two initials, followed by the
265 two digits 10, at the domain cam.ac.uk.
266 .
267 .
269 .rs
270 .sp
271 .nf
272 Last updated: 07 August 2007
273 Copyright (c) 1997-2007 University of Cambridge.
274 .fi


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