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Wed Aug 8 14:24:50 2007 UTC (14 years, 2 months ago) by ph10
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Add Perl 5.10's backtracking verbs.
3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 This document describes the differences in the ways that PCRE and Perl handle
8 regular expressions. The differences described here are mainly with respect to
9 Perl 5.8, though PCRE versions 7.0 and later contain some features that are
10 expected to be in the forthcoming Perl 5.10.
11 .P
12 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
13 it does have are given in the
14 .\" HTML <a href="pcre.html#utf8support">
15 .\" </a>
16 section on UTF-8 support
17 .\"
18 in the main
19 .\" HREF
20 \fBpcre\fP
21 .\"
22 page.
23 .P
24 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
25 them, but they do not mean what you might think. For example, (?!a){3} does
26 not assert that the next three characters are not "a". It just asserts that the
27 next character is not "a" three times.
28 .P
29 3. Capturing subpatterns that occur inside negative lookahead assertions are
30 counted, but their entries in the offsets vector are never set. Perl sets its
31 numerical variables from any such patterns that are matched before the
32 assertion fails to match something (thereby succeeding), but only if the
33 negative lookahead assertion contains just one branch.
34 .P
35 4. Though binary zero characters are supported in the subject string, they are
36 not allowed in a pattern string because it is passed as a normal C string,
37 terminated by zero. The escape sequence \e0 can be used in the pattern to
38 represent a binary zero.
39 .P
40 5. The following Perl escape sequences are not supported: \el, \eu, \eL,
41 \eU, and \eN. In fact these are implemented by Perl's general string-handling
42 and are not part of its pattern matching engine. If any of these are
43 encountered by PCRE, an error is generated.
44 .P
45 6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
46 built with Unicode character property support. The properties that can be
47 tested with \ep and \eP are limited to the general category properties such as
48 Lu and Nd, script names such as Greek or Han, and the derived properties Any
49 and L&.
50 .P
51 7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
52 between are treated as literals. This is slightly different from Perl in that $
53 and @ are also handled as literals inside the quotes. In Perl, they cause
54 variable interpolation (but of course PCRE does not have variables). Note the
55 following examples:
56 .sp
57 Pattern PCRE matches Perl matches
58 .sp
59 .\" JOIN
60 \eQabc$xyz\eE abc$xyz abc followed by the
61 contents of $xyz
62 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
63 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
64 .sp
65 The \eQ...\eE sequence is recognized both inside and outside character classes.
66 .P
67 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
68 constructions. However, there is support for recursive patterns. This is not
69 available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
70 feature allows an external function to be called during pattern matching. See
71 the
72 .\" HREF
73 \fBpcrecallout\fP
74 .\"
75 documentation for details.
76 .P
77 9. Subpatterns that are called recursively or as "subroutines" are always
78 treated as atomic groups in PCRE. This is like Python, but unlike Perl.
79 .P
80 10. There are some differences that are concerned with the settings of captured
81 strings when part of a pattern is repeated. For example, matching "aba" against
82 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
83 .P
84 11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
85 (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
86 argument. PCRE does not support (*MARK). If (*ACCEPT) is within capturing
87 parentheses, PCRE does not set that capture group; this is different to Perl.
88 .P
89 12. PCRE provides some extensions to the Perl regular expression facilities.
90 Perl 5.10 will include new features that are not in earlier versions, some of
91 which (such as named parentheses) have been in PCRE for some time. This list is
92 with respect to Perl 5.10:
93 .sp
94 (a) Although lookbehind assertions must match fixed length strings, each
95 alternative branch of a lookbehind assertion can match a different length of
96 string. Perl requires them all to have the same length.
97 .sp
98 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
99 meta-character matches only at the very end of the string.
100 .sp
101 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
102 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
103 (Perl can be made to issue a warning.)
104 .sp
105 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
106 inverted, that is, by default they are not greedy, but if followed by a
107 question mark they are.
108 .sp
109 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
110 only at the first matching position in the subject string.
111 .sp
113 options for \fBpcre_exec()\fP have no Perl equivalents.
114 .sp
115 (g) The callout facility is PCRE-specific.
116 .sp
117 (h) The partial matching facility is PCRE-specific.
118 .sp
119 (i) Patterns compiled by PCRE can be saved and re-used at a later time, even on
120 different hosts that have the other endianness.
121 .sp
122 (j) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
123 different way and is not Perl-compatible.
124 .
125 .
127 .rs
128 .sp
129 .nf
130 Philip Hazel
131 University Computing Service
132 Cambridge CB2 3QH, England.
133 .fi
134 .
135 .
137 .rs
138 .sp
139 .nf
140 Last updated: 08 August 2007
141 Copyright (c) 1997-2007 University of Cambridge.
142 .fi


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