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Fri Oct 2 08:53:31 2009 UTC (11 years, 10 months ago) by ph10
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Documentation update
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 with respect to Perl
9 5.10.
10 .P
11 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
12 it does have are given in the
13 .\" HTML <a href="pcre.html#utf8support">
14 .\" </a>
15 section on UTF-8 support
16 .\"
17 in the main
18 .\" HREF
19 \fBpcre\fP
20 .\"
21 page.
22 .P
23 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
24 them, but they do not mean what you might think. For example, (?!a){3} does
25 not assert that the next three characters are not "a". It just asserts that the
26 next character is not "a" three times.
27 .P
28 3. Capturing subpatterns that occur inside negative lookahead assertions are
29 counted, but their entries in the offsets vector are never set. Perl sets its
30 numerical variables from any such patterns that are matched before the
31 assertion fails to match something (thereby succeeding), but only if the
32 negative lookahead assertion contains just one branch.
33 .P
34 4. Though binary zero characters are supported in the subject string, they are
35 not allowed in a pattern string because it is passed as a normal C string,
36 terminated by zero. The escape sequence \e0 can be used in the pattern to
37 represent a binary zero.
38 .P
39 5. The following Perl escape sequences are not supported: \el, \eu, \eL,
40 \eU, and \eN. In fact these are implemented by Perl's general string-handling
41 and are not part of its pattern matching engine. If any of these are
42 encountered by PCRE, an error is generated.
43 .P
44 6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
45 built with Unicode character property support. The properties that can be
46 tested with \ep and \eP are limited to the general category properties such as
47 Lu and Nd, script names such as Greek or Han, and the derived properties Any
48 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
49 Perl documentation says "Because Perl hides the need for the user to understand
50 the internal representation of Unicode characters, there is no need to
51 implement the somewhat messy concept of surrogates."
52 .P
53 7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
54 between are treated as literals. This is slightly different from Perl in that $
55 and @ are also handled as literals inside the quotes. In Perl, they cause
56 variable interpolation (but of course PCRE does not have variables). Note the
57 following examples:
58 .sp
59 Pattern PCRE matches Perl matches
60 .sp
61 .\" JOIN
62 \eQabc$xyz\eE abc$xyz abc followed by the
63 contents of $xyz
64 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
65 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
66 .sp
67 The \eQ...\eE sequence is recognized both inside and outside character classes.
68 .P
69 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
70 constructions. However, there is support for recursive patterns. This is not
71 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
72 feature allows an external function to be called during pattern matching. See
73 the
74 .\" HREF
75 \fBpcrecallout\fP
76 .\"
77 documentation for details.
78 .P
79 9. Subpatterns that are called recursively or as "subroutines" are always
80 treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
81 is a discussion of an example that explains this in more detail in the
82 .\" HTML <a href="pcrepattern.html#recursiondifference">
83 .\" </a>
84 section on recursion differences from Perl
85 .\"
86 in the
87 .\" HREF
88 \fBpcrepattern\fP
89 .\"
90 page.
91 .P
92 10. There are some differences that are concerned with the settings of captured
93 strings when part of a pattern is repeated. For example, matching "aba" against
94 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
95 .P
96 11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
97 (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
98 argument. PCRE does not support (*MARK).
99 .P
100 12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
101 names is not as general as Perl's. This is a consequence of the fact the PCRE
102 works internally just with numbers, using an external table to translate
103 between numbers and names. The following are some specific differences:
104 .sp
105 (a) After matching a pattern such as (?|(?<a>A)|(?<b)B) where the two capturing
106 parentheses have the same number but different names, it is not possible to
107 distinguish which parentheses matched, because both names map to capturing
108 subpattern number 1.
109 .sp
110 (b) A condition test for a subpattern with a name that is duplicated gives
111 unpredictable results. For example, when the pattern
112 (?:(?<a>A)|(?<a>B))(?('a')...|...) is compiled (the PCRE_DUPNAMES option is
113 required), the condition test (?('a') is set to test whether subpattern 1 has
114 matched, ignoring subpattern 2, even though it has the same name.
115 .P
116 13. PCRE provides some extensions to the Perl regular expression facilities.
117 Perl 5.10 includes new features that are not in earlier versions of Perl, some
118 of which (such as named parentheses) have been in PCRE for some time. This list
119 is with respect to Perl 5.10:
120 .sp
121 (a) Although lookbehind assertions in PCRE must match fixed length strings,
122 each alternative branch of a lookbehind assertion can match a different length
123 of string. Perl requires them all to have the same length.
124 .sp
125 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
126 meta-character matches only at the very end of the string.
127 .sp
128 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
129 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
130 (Perl can be made to issue a warning.)
131 .sp
132 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
133 inverted, that is, by default they are not greedy, but if followed by a
134 question mark they are.
135 .sp
136 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
137 only at the first matching position in the subject string.
138 .sp
140 PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
141 .sp
142 (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
143 by the PCRE_BSR_ANYCRLF option.
144 .sp
145 (h) The callout facility is PCRE-specific.
146 .sp
147 (i) The partial matching facility is PCRE-specific.
148 .sp
149 (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
150 different hosts that have the other endianness.
151 .sp
152 (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
153 different way and is not Perl-compatible.
154 .sp
155 (l) PCRE recognizes some special sequences such as (*CR) at the start of
156 a pattern that set overall options that cannot be changed within the pattern.
157 .
158 .
160 .rs
161 .sp
162 .nf
163 Philip Hazel
164 University Computing Service
165 Cambridge CB2 3QH, England.
166 .fi
167 .
168 .
170 .rs
171 .sp
172 .nf
173 Last updated: 29 September 2009
174 Copyright (c) 1997-2009 University of Cambridge.
175 .fi


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