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Sat Jul 23 14:34:27 2011 UTC (10 years, 2 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 versions 5.10 and above.
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 assertions. Perl permits them, but
24 they do not mean what you might think. For example, (?!a){3} does not assert
25 that the next three characters are not "a". It just asserts that the next
26 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 and \eU. In fact these are implemented by Perl's general string-handling and
41 are not part of its pattern matching engine. If any of these are encountered by
42 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 implements a simpler version of \eX than Perl, which changed to make
54 \eX match what Unicode calls an "extended grapheme cluster". This is more
55 complicated than an extended Unicode sequence, which is what PCRE matches.
56 .P
57 8. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
58 between are treated as literals. This is slightly different from Perl in that $
59 and @ are also handled as literals inside the quotes. In Perl, they cause
60 variable interpolation (but of course PCRE does not have variables). Note the
61 following examples:
62 .sp
63 Pattern PCRE matches Perl matches
64 .sp
65 .\" JOIN
66 \eQabc$xyz\eE abc$xyz abc followed by the
67 contents of $xyz
68 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
69 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
70 .sp
71 The \eQ...\eE sequence is recognized both inside and outside character classes.
72 .P
73 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
74 constructions. However, there is support for recursive patterns. This is not
75 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
76 feature allows an external function to be called during pattern matching. See
77 the
78 .\" HREF
79 \fBpcrecallout\fP
80 .\"
81 documentation for details.
82 .P
83 10. Subpatterns that are called recursively or as "subroutines" are always
84 treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
85 is a discussion of an example that explains this in more detail in the
86 .\" HTML <a href="pcrepattern.html#recursiondifference">
87 .\" </a>
88 section on recursion differences from Perl
89 .\"
90 in the
91 .\" HREF
92 \fBpcrepattern\fP
93 .\"
94 page.
95 .P
96 11. There are some differences that are concerned with the settings of captured
97 strings when part of a pattern is repeated. For example, matching "aba" against
98 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
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. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
104 where the two capturing parentheses have the same number but different names,
105 is not supported, and causes an error at compile time. If it were allowed, it
106 would not be possible to distinguish which parentheses matched, because both
107 names map to capturing subpattern number 1. To avoid this confusing situation,
108 an error is given at compile time.
109 .P
110 13. Perl recognizes comments in some places that PCRE doesn't, for example,
111 between the ( and ? at the start of a subpattern.
112 .P
113 14. PCRE provides some extensions to the Perl regular expression facilities.
114 Perl 5.10 includes new features that are not in earlier versions of Perl, some
115 of which (such as named parentheses) have been in PCRE for some time. This list
116 is with respect to Perl 5.10:
117 .sp
118 (a) Although lookbehind assertions in PCRE must match fixed length strings,
119 each alternative branch of a lookbehind assertion can match a different length
120 of string. Perl requires them all to have the same length.
121 .sp
122 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
123 meta-character matches only at the very end of the string.
124 .sp
125 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
126 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
127 (Perl can be made to issue a warning.)
128 .sp
129 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
130 inverted, that is, by default they are not greedy, but if followed by a
131 question mark they are.
132 .sp
133 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
134 only at the first matching position in the subject string.
135 .sp
137 PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
138 .sp
139 (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
140 by the PCRE_BSR_ANYCRLF option.
141 .sp
142 (h) The callout facility is PCRE-specific.
143 .sp
144 (i) The partial matching facility is PCRE-specific.
145 .sp
146 (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
147 different hosts that have the other endianness.
148 .sp
149 (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
150 different way and is not Perl-compatible.
151 .sp
152 (l) PCRE recognizes some special sequences such as (*CR) at the start of
153 a pattern that set overall options that cannot be changed within the pattern.
154 .
155 .
157 .rs
158 .sp
159 .nf
160 Philip Hazel
161 University Computing Service
162 Cambridge CB2 3QH, England.
163 .fi
164 .
165 .
167 .rs
168 .sp
169 .nf
170 Last updated: 23 July 2011
171 Copyright (c) 1997-2011 University of Cambridge.
172 .fi


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