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Revision 903 - (show annotations)
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Source file tidies for 8.30-RC1 release; fix Makefile.am bugs for building 
symbolic links to man pages.
1 .TH PCREUNICODE 3
2 .SH NAME
3 PCRE - Perl-compatible regular expressions
4 .SH "UTF-8, UTF-16, AND UNICODE PROPERTY SUPPORT"
5 .rs
6 .sp
7 From Release 8.30, in addition to its previous UTF-8 support, PCRE also
8 supports UTF-16 by means of a separate 16-bit library. This can be built as
9 well as, or instead of, the 8-bit library.
10 .
11 .
12 .SH "UTF-8 SUPPORT"
13 .rs
14 .sp
15 In order process UTF-8 strings, you must build PCRE's 8-bit library with UTF
16 support, and, in addition, you must call
17 .\" HREF
18 \fBpcre_compile()\fP
19 .\"
20 with the PCRE_UTF8 option flag, or the pattern must start with the sequence
21 (*UTF8). When either of these is the case, both the pattern and any subject
22 strings that are matched against it are treated as UTF-8 strings instead of
23 strings of 1-byte characters.
24 .
25 .
26 .SH "UTF-16 SUPPORT"
27 .rs
28 .sp
29 In order process UTF-16 strings, you must build PCRE's 16-bit library with UTF
30 support, and, in addition, you must call
31 .\" HTML <a href="pcre_compile.html">
32 .\" </a>
33 \fBpcre16_compile()\fP
34 .\"
35 with the PCRE_UTF16 option flag, or the pattern must start with the sequence
36 (*UTF16). When either of these is the case, both the pattern and any subject
37 strings that are matched against it are treated as UTF-16 strings instead of
38 strings of 16-bit characters.
39 .
40 .
41 .SH "UTF SUPPORT OVERHEAD"
42 .rs
43 .sp
44 If you compile PCRE with UTF support, but do not use it at run time, the
45 library will be a bit bigger, but the additional run time overhead is limited
46 to testing the PCRE_UTF8/16 flag occasionally, so should not be very big.
47 .
48 .
49 .SH "UNICODE PROPERTY SUPPORT"
50 .rs
51 .sp
52 If PCRE is built with Unicode character property support (which implies UTF
53 support), the escape sequences \ep{..}, \eP{..}, and \eX can be used.
54 The available properties that can be tested are limited to the general
55 category properties such as Lu for an upper case letter or Nd for a decimal
56 number, the Unicode script names such as Arabic or Han, and the derived
57 properties Any and L&. A full list is given in the
58 .\" HREF
59 \fBpcrepattern\fP
60 .\"
61 documentation. Only the short names for properties are supported. For example,
62 \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
63 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
64 compatibility with Perl 5.6. PCRE does not support this.
65 .
66 .
67 .\" HTML <a name="utf8strings"></a>
68 .SS "Validity of UTF-8 strings"
69 .rs
70 .sp
71 When you set the PCRE_UTF8 flag, the byte strings passed as patterns and
72 subjects are (by default) checked for validity on entry to the relevant
73 functions. From release 7.3 of PCRE, the check is according the rules of RFC
74 3629, which are themselves derived from the Unicode specification. Earlier
75 releases of PCRE followed the rules of RFC 2279, which allows the full range of
76 31-bit values (0 to 0x7FFFFFFF). The current check allows only values in the
77 range U+0 to U+10FFFF, excluding U+D800 to U+DFFF.
78 .P
79 The excluded code points are the "Surrogate Area" of Unicode. They are reserved
80 for use by UTF-16, where they are used in pairs to encode codepoints with
81 values greater than 0xFFFF. The code points that are encoded by UTF-16 pairs
82 are available independently in the UTF-8 encoding. (In other words, the whole
83 surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
84 .P
85 If an invalid UTF-8 string is passed to PCRE, an error return is given. At
86 compile time, the only additional information is the offset to the first byte
87 of the failing character. The runtime functions \fBpcre_exec()\fP and
88 \fBpcre_dfa_exec()\fP also pass back this information, as well as a more
89 detailed reason code if the caller has provided memory in which to do this.
90 .P
91 In some situations, you may already know that your strings are valid, and
92 therefore want to skip these checks in order to improve performance. If you set
93 the PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes that
94 the pattern or subject it is given (respectively) contains only valid UTF-8
95 codes. In this case, it does not diagnose an invalid UTF-8 string.
96 .P
97 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
98 happens depends on why the string is invalid. If the string conforms to the
99 "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
100 in the range 0 to 0x7FFFFFFF by \fBpcre_dfa_exec()\fP and the interpreted
101 version of \fBpcre_exec()\fP. In other words, apart from the initial validity
102 test, these functions (when in UTF-8 mode) handle strings according to the more
103 liberal rules of RFC 2279. However, the just-in-time (JIT) optimization for
104 \fBpcre_exec()\fP supports only RFC 3629. If you are using JIT optimization, or
105 if the string does not even conform to RFC 2279, the result is undefined. Your
106 program may crash.
107 .P
108 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
109 encoded in a UTF-8-like manner as per the old RFC, you can set
110 PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
111 situation, you will have to apply your own validity check, and avoid the use of
112 JIT optimization.
113 .
114 .
115 .\" HTML <a name="utf16strings"></a>
116 .SS "Validity of UTF-16 strings"
117 .rs
118 .sp
119 When you set the PCRE_UTF16 flag, the strings of 16-bit data units that are
120 passed as patterns and subjects are (by default) checked for validity on entry
121 to the relevant functions. Values other than those in the surrogate range
122 U+D800 to U+DFFF are independent code points. Values in the surrogate range
123 must be used in pairs in the correct manner.
124 .P
125 If an invalid UTF-16 string is passed to PCRE, an error return is given. At
126 compile time, the only additional information is the offset to the first data
127 unit of the failing character. The runtime functions \fBpcre16_exec()\fP and
128 \fBpcre16_dfa_exec()\fP also pass back this information, as well as a more
129 detailed reason code if the caller has provided memory in which to do this.
130 .P
131 In some situations, you may already know that your strings are valid, and
132 therefore want to skip these checks in order to improve performance. If you set
133 the PCRE_NO_UTF16_CHECK flag at compile time or at run time, PCRE assumes that
134 the pattern or subject it is given (respectively) contains only valid UTF-16
135 sequences. In this case, it does not diagnose an invalid UTF-16 string.
136 .
137 .
138 .SS "General comments about UTF modes"
139 .rs
140 .sp
141 1. Codepoints less than 256 can be specified by either braced or unbraced
142 hexadecimal escape sequences (for example, \ex{b3} or \exb3). Larger values
143 have to use braced sequences.
144 .P
145 2. Octal numbers up to \e777 are recognized, and in UTF-8 mode, they match
146 two-byte characters for values greater than \e177.
147 .P
148 3. Repeat quantifiers apply to complete UTF characters, not to individual
149 data units, for example: \ex{100}{3}.
150 .P
151 4. The dot metacharacter matches one UTF character instead of a single data
152 unit.
153 .P
154 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode, or
155 a single 16-bit data unit in UTF-16 mode, but its use can lead to some strange
156 effects because it breaks up multi-unit characters (see the description of \eC
157 in the
158 .\" HREF
159 \fBpcrepattern\fP
160 .\"
161 documentation). The use of \eC is not supported in the alternative matching
162 function \fBpcre[16]_dfa_exec()\fP, nor is it supported in UTF mode by the JIT
163 optimization of \fBpcre[16]_exec()\fP. If JIT optimization is requested for a
164 UTF pattern that contains \eC, it will not succeed, and so the matching will
165 be carried out by the normal interpretive function.
166 .P
167 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
168 test characters of any code value, but, by default, the characters that PCRE
169 recognizes as digits, spaces, or word characters remain the same set as in
170 non-UTF mode, all with values less than 256. This remains true even when PCRE
171 is built to include Unicode property support, because to do otherwise would
172 slow down PCRE in many common cases. Note in particular that this applies to
173 \eb and \eB, because they are defined in terms of \ew and \eW. If you really
174 want to test for a wider sense of, say, "digit", you can use explicit Unicode
175 property tests such as \ep{Nd}. Alternatively, if you set the PCRE_UCP option,
176 the way that the character escapes work is changed so that Unicode properties
177 are used to determine which characters match. There are more details in the
178 section on
179 .\" HTML <a href="pcrepattern.html#genericchartypes">
180 .\" </a>
181 generic character types
182 .\"
183 in the
184 .\" HREF
185 \fBpcrepattern\fP
186 .\"
187 documentation.
188 .P
189 7. Similarly, characters that match the POSIX named character classes are all
190 low-valued characters, unless the PCRE_UCP option is set.
191 .P
192 8. However, the horizontal and vertical whitespace matching escapes (\eh, \eH,
193 \ev, and \eV) do match all the appropriate Unicode characters, whether or not
194 PCRE_UCP is set.
195 .P
196 9. Case-insensitive matching applies only to characters whose values are less
197 than 128, unless PCRE is built with Unicode property support. Even when Unicode
198 property support is available, PCRE still uses its own character tables when
199 checking the case of low-valued characters, so as not to degrade performance.
200 The Unicode property information is used only for characters with higher
201 values. Furthermore, PCRE supports case-insensitive matching only when there is
202 a one-to-one mapping between a letter's cases. There are a small number of
203 many-to-one mappings in Unicode; these are not supported by PCRE.
204 .
205 .
206 .SH AUTHOR
207 .rs
208 .sp
209 .nf
210 Philip Hazel
211 University Computing Service
212 Cambridge CB2 3QH, England.
213 .fi
214 .
215 .
216 .SH REVISION
217 .rs
218 .sp
219 .nf
220 Last updated: 13 January 2012
221 Copyright (c) 1997-2012 University of Cambridge.
222 .fi

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