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Revision 868 - (show annotations)
Fri Jan 13 17:16:32 2012 UTC (8 years, 1 month ago) by ph10
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The last of the 16-bit documentation major updates.
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 .\" HREF
32 \fBpcre16_compile()\fP
33 .\"
34 with the PCRE_UTF16 option flag, or the pattern must start with the sequence
35 (*UTF16). When either of these is the case, both the pattern and any subject
36 strings that are matched against it are treated as UTF-16 strings instead of
37 strings of 16-bit characters.
38 .
39 .
40 .SH "UTF SUPPORT OVERHEAD"
41 .rs
42 .sp
43 If you compile PCRE with UTF support, but do not use it at run time, the
44 library will be a bit bigger, but the additional run time overhead is limited
45 to testing the PCRE_UTF8/16 flag occasionally, so should not be very big.
46 .
47 .
48 .SH "UNICODE PROPERTY SUPPORT"
49 .rs
50 .sp
51 If PCRE is built with Unicode character property support (which implies UTF
52 support), the escape sequences \ep{..}, \eP{..}, and \eX can be used.
53 The available properties that can be tested are limited to the general
54 category properties such as Lu for an upper case letter or Nd for a decimal
55 number, the Unicode script names such as Arabic or Han, and the derived
56 properties Any and L&. A full list is given in the
57 .\" HREF
58 \fBpcrepattern\fP
59 .\"
60 documentation. Only the short names for properties are supported. For example,
61 \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
62 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
63 compatibility with Perl 5.6. PCRE does not support this.
64 .
65 .
66 .\" HTML <a name="utf8strings"></a>
67 .SS "Validity of UTF-8 strings"
68 .rs
69 .sp
70 When you set the PCRE_UTF8 flag, the byte strings passed as patterns and
71 subjects are (by default) checked for validity on entry to the relevant
72 functions. From release 7.3 of PCRE, the check is according the rules of RFC
73 3629, which are themselves derived from the Unicode specification. Earlier
74 releases of PCRE followed the rules of RFC 2279, which allows the full range of
75 31-bit values (0 to 0x7FFFFFFF). The current check allows only values in the
76 range U+0 to U+10FFFF, excluding U+D800 to U+DFFF.
77 .P
78 The excluded code points are the "Surrogate Area" of Unicode. They are reserved
79 for use by UTF-16, where they are used in pairs to encode codepoints with
80 values greater than 0xFFFF. The code points that are encoded by UTF-16 pairs
81 are available independently in the UTF-8 encoding. (In other words, the whole
82 surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
83 .P
84 If an invalid UTF-8 string is passed to PCRE, an error return is given. At
85 compile time, the only additional information is the offset to the first byte
86 of the failing character. The runtime functions \fBpcre_exec()\fP and
87 \fBpcre_dfa_exec()\fP also pass back this information, as well as a more
88 detailed reason code if the caller has provided memory in which to do this.
89 .P
90 In some situations, you may already know that your strings are valid, and
91 therefore want to skip these checks in order to improve performance. If you set
92 the PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes that
93 the pattern or subject it is given (respectively) contains only valid UTF-8
94 codes. In this case, it does not diagnose an invalid UTF-8 string.
95 .P
96 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
97 happens depends on why the string is invalid. If the string conforms to the
98 "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
99 in the range 0 to 0x7FFFFFFF by \fBpcre_dfa_exec()\fP and the interpreted
100 version of \fBpcre_exec()\fP. In other words, apart from the initial validity
101 test, these functions (when in UTF-8 mode) handle strings according to the more
102 liberal rules of RFC 2279. However, the just-in-time (JIT) optimization for
103 \fBpcre_exec()\fP supports only RFC 3629. If you are using JIT optimization, or
104 if the string does not even conform to RFC 2279, the result is undefined. Your
105 program may crash.
106 .P
107 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
108 encoded in a UTF-8-like manner as per the old RFC, you can set
109 PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
110 situation, you will have to apply your own validity check, and avoid the use of
111 JIT optimization.
112 .
113 .
114 .\" HTML <a name="utf16strings"></a>
115 .SS "Validity of UTF-16 strings"
116 .rs
117 .sp
118 When you set the PCRE_UTF16 flag, the strings of 16-bit data units that are
119 passed as patterns and subjects are (by default) checked for validity on entry
120 to the relevant functions. Values other than those in the surrogate range
121 U+D800 to U+DFFF are independent code points. Values in the surrogate range
122 must be used in pairs in the correct manner.
123 .P
124 If an invalid UTF-16 string is passed to PCRE, an error return is given. At
125 compile time, the only additional information is the offset to the first data
126 unit of the failing character. The runtime functions \fBpcre16_exec()\fP and
127 \fBpcre16_dfa_exec()\fP also pass back this information, as well as a more
128 detailed reason code if the caller has provided memory in which to do this.
129 .P
130 In some situations, you may already know that your strings are valid, and
131 therefore want to skip these checks in order to improve performance. If you set
132 the PCRE_NO_UTF16_CHECK flag at compile time or at run time, PCRE assumes that
133 the pattern or subject it is given (respectively) contains only valid UTF-16
134 sequences. In this case, it does not diagnose an invalid UTF-16 string.
135 .
136 .
137 .SS "General comments about UTF modes"
138 .rs
139 .sp
140 1. Codepoints less than 256 can be specified by either braced or unbraced
141 hexadecimal escape sequences (for example, \ex{b3} or \exb3). Larger values
142 have to use braced sequences.
143 .P
144 2. Octal numbers up to \e777 are recognized, and in UTF-8 mode, they match
145 two-byte characters for values greater than \e177.
146 .P
147 3. Repeat quantifiers apply to complete UTF characters, not to individual
148 data units, for example: \ex{100}{3}.
149 .P
150 4. The dot metacharacter matches one UTF character instead of a single data
151 unit.
152 .P
153 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode, or
154 a single 16-bit data unit in UTF-16 mode, but its use can lead to some strange
155 effects because it breaks up multi-unit characters (see the description of \eC
156 in the
157 .\" HREF
158 \fBpcrepattern\fP
159 .\"
160 documentation). The use of \eC is not supported in the alternative matching
161 function \fBpcre[16]_dfa_exec()\fP, nor is it supported in UTF mode by the JIT
162 optimization of \fBpcre[16]_exec()\fP. If JIT optimization is requested for a
163 UTF pattern that contains \eC, it will not succeed, and so the matching will
164 be carried out by the normal interpretive function.
165 .P
166 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
167 test characters of any code value, but, by default, the characters that PCRE
168 recognizes as digits, spaces, or word characters remain the same set as in
169 non-UTF mode, all with values less than 256. This remains true even when PCRE
170 is built to include Unicode property support, because to do otherwise would
171 slow down PCRE in many common cases. Note in particular that this applies to
172 \eb and \eB, because they are defined in terms of \ew and \eW. If you really
173 want to test for a wider sense of, say, "digit", you can use explicit Unicode
174 property tests such as \ep{Nd}. Alternatively, if you set the PCRE_UCP option,
175 the way that the character escapes work is changed so that Unicode properties
176 are used to determine which characters match. There are more details in the
177 section on
178 .\" HTML <a href="pcrepattern.html#genericchartypes">
179 .\" </a>
180 generic character types
181 .\"
182 in the
183 .\" HREF
184 \fBpcrepattern\fP
185 .\"
186 documentation.
187 .P
188 7. Similarly, characters that match the POSIX named character classes are all
189 low-valued characters, unless the PCRE_UCP option is set.
190 .P
191 8. However, the horizontal and vertical whitespace matching escapes (\eh, \eH,
192 \ev, and \eV) do match all the appropriate Unicode characters, whether or not
193 PCRE_UCP is set.
194 .P
195 9. Case-insensitive matching applies only to characters whose values are less
196 than 128, unless PCRE is built with Unicode property support. Even when Unicode
197 property support is available, PCRE still uses its own character tables when
198 checking the case of low-valued characters, so as not to degrade performance.
199 The Unicode property information is used only for characters with higher
200 values. Furthermore, PCRE supports case-insensitive matching only when there is
201 a one-to-one mapping between a letter's cases. There are a small number of
202 many-to-one mappings in Unicode; these are not supported by PCRE.
203 .
204 .
205 .SH AUTHOR
206 .rs
207 .sp
208 .nf
209 Philip Hazel
210 University Computing Service
211 Cambridge CB2 3QH, England.
212 .fi
213 .
214 .
215 .SH REVISION
216 .rs
217 .sp
218 .nf
219 Last updated: 13 January 2012
220 Copyright (c) 1997-2012 University of Cambridge.
221 .fi

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