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Tue Sep 6 10:37:15 2011 UTC (9 years, 10 months ago) by ph10
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Documentation updates.
3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
8 the code, and, in addition, you must call
9 .\" HREF
10 \fBpcre_compile()\fP
11 .\"
12 with the PCRE_UTF8 option flag, or the pattern must start with the sequence
13 (*UTF8). When either of these is the case, both the pattern and any subject
14 strings that are matched against it are treated as UTF-8 strings instead of
15 strings of 1-byte characters. PCRE does not support any other formats (in
16 particular, it does not support UTF-16).
17 .P
18 If you compile PCRE with UTF-8 support, but do not use it at run time, the
19 library will be a bit bigger, but the additional run time overhead is limited
20 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
21 .P
22 If PCRE is built with Unicode character property support (which implies UTF-8
23 support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
24 The available properties that can be tested are limited to the general
25 category properties such as Lu for an upper case letter or Nd for a decimal
26 number, the Unicode script names such as Arabic or Han, and the derived
27 properties Any and L&. A full list is given in the
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29 \fBpcrepattern\fP
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31 documentation. Only the short names for properties are supported. For example,
32 \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
33 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
34 compatibility with Perl 5.6. PCRE does not support this.
35 .
36 .
37 .\" HTML <a name="utf8strings"></a>
38 .SS "Validity of UTF-8 strings"
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40 .sp
41 When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
42 are (by default) checked for validity on entry to the relevant functions. From
43 release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
44 themselves derived from the Unicode specification. Earlier releases of PCRE
45 followed the rules of RFC 2279, which allows the full range of 31-bit values (0
46 to 0x7FFFFFFF). The current check allows only values in the range U+0 to
47 U+10FFFF, excluding U+D800 to U+DFFF.
48 .P
49 The excluded code points are the "Low Surrogate Area" of Unicode, of which the
50 Unicode Standard says this: "The Low Surrogate Area does not contain any
51 character assignments, consequently no character code charts or namelists are
52 provided for this area. Surrogates are reserved for use with UTF-16 and then
53 must be used in pairs." The code points that are encoded by UTF-16 pairs are
54 available as independent code points in the UTF-8 encoding. (In other words,
55 the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
56 UTF-8.)
57 .P
58 If an invalid UTF-8 string is passed to PCRE, an error return is given. At
59 compile time, the only additional information is the offset to the first byte
60 of the failing character. The runtime functions \fBpcre_exec()\fP and
61 \fBpcre_dfa_exec()\fP also pass back this information, as well as a more
62 detailed reason code if the caller has provided memory in which to do this.
63 .P
64 In some situations, you may already know that your strings are valid, and
65 therefore want to skip these checks in order to improve performance. If you set
66 the PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes that
67 the pattern or subject it is given (respectively) contains only valid UTF-8
68 codes. In this case, it does not diagnose an invalid UTF-8 string.
69 .P
70 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
71 happens depends on why the string is invalid. If the string conforms to the
72 "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
73 in the range 0 to 0x7FFFFFFF by \fBpcre_dfa_exec()\fP and the interpreted
74 version of \fBpcre_exec()\fP. In other words, apart from the initial validity
75 test, these functions (when in UTF-8 mode) handle strings according to the more
76 liberal rules of RFC 2279. However, the just-in-time (JIT) optimization for
77 \fBpcre_exec()\fP supports only RFC 3629. If you are using JIT optimization, or
78 if the string does not even conform to RFC 2279, the result is undefined. Your
79 program may crash.
80 .P
81 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
82 encoded in a UTF-8-like manner as per the old RFC, you can set
83 PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
84 situation, you will have to apply your own validity check, and avoid the use of
85 JIT optimization.
86 .
87 .
88 .SS "General comments about UTF-8 mode"
89 .rs
90 .sp
91 1. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
92 UTF-8 character if the value is greater than 127.
93 .P
94 2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
95 characters for values greater than \e177.
96 .P
97 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
98 bytes, for example: \ex{100}{3}.
99 .P
100 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
101 .P
102 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
103 but its use can lead to some strange effects. This facility is not available in
104 the alternative matching function, \fBpcre_dfa_exec()\fP, nor is it supported
105 by the JIT optimization of \fBpcre_exec()\fP. If JIT optimization is requested
106 for a pattern that contains \eC, it will not succeed, and so the matching will
107 be carried out by the normal interpretive function.
108 .P
109 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
110 test characters of any code value, but, by default, the characters that PCRE
111 recognizes as digits, spaces, or word characters remain the same set as before,
112 all with values less than 256. This remains true even when PCRE is built to
113 include Unicode property support, because to do otherwise would slow down PCRE
114 in many common cases. Note in particular that this applies to \eb and \eB,
115 because they are defined in terms of \ew and \eW. If you really want to test
116 for a wider sense of, say, "digit", you can use explicit Unicode property tests
117 such as \ep{Nd}. Alternatively, if you set the PCRE_UCP option, the way that
118 the character escapes work is changed so that Unicode properties are used to
119 determine which characters match. There are more details in the section on
120 .\" HTML <a href="pcrepattern.html#genericchartypes">
121 .\" </a>
122 generic character types
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124 in the
125 .\" HREF
126 \fBpcrepattern\fP
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128 documentation.
129 .P
130 7. Similarly, characters that match the POSIX named character classes are all
131 low-valued characters, unless the PCRE_UCP option is set.
132 .P
133 8. However, the horizontal and vertical whitespace matching escapes (\eh, \eH,
134 \ev, and \eV) do match all the appropriate Unicode characters, whether or not
135 PCRE_UCP is set.
136 .P
137 9. Case-insensitive matching applies only to characters whose values are less
138 than 128, unless PCRE is built with Unicode property support. Even when Unicode
139 property support is available, PCRE still uses its own character tables when
140 checking the case of low-valued characters, so as not to degrade performance.
141 The Unicode property information is used only for characters with higher
142 values. Furthermore, PCRE supports case-insensitive matching only when there is
143 a one-to-one mapping between a letter's cases. There are a small number of
144 many-to-one mappings in Unicode; these are not supported by PCRE.
145 .
146 .
148 .rs
149 .sp
150 .nf
151 Philip Hazel
152 University Computing Service
153 Cambridge CB2 3QH, England.
154 .fi
155 .
156 .
158 .rs
159 .sp
160 .nf
161 Last updated: 06 September 2011
162 Copyright (c) 1997-2011 University of Cambridge.
163 .fi

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