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Better documentation patch.
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 \eU, and \eN when followed by a character name or Unicode value. (\eN on its
41 own, matching a non-newline character, is supported.) In fact these are
42 implemented by Perl's general string-handling and are not part of its pattern
43 matching engine. If any of these are encountered by PCRE, an error is
44 generated.
45 .P
46 6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
47 built with Unicode character property support. The properties that can be
48 tested with \ep and \eP are limited to the general category properties such as
49 Lu and Nd, script names such as Greek or Han, and the derived properties Any
50 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
51 Perl documentation says "Because Perl hides the need for the user to understand
52 the internal representation of Unicode characters, there is no need to
53 implement the somewhat messy concept of surrogates."
54 .P
55 7. PCRE implements a simpler version of \eX than Perl, which changed to make
56 \eX match what Unicode calls an "extended grapheme cluster". This is more
57 complicated than an extended Unicode sequence, which is what PCRE matches.
58 .P
59 8. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
60 between are treated as literals. This is slightly different from Perl in that $
61 and @ are also handled as literals inside the quotes. In Perl, they cause
62 variable interpolation (but of course PCRE does not have variables). Note the
63 following examples:
64 .sp
65 Pattern PCRE matches Perl matches
66 .sp
67 .\" JOIN
68 \eQabc$xyz\eE abc$xyz abc followed by the
69 contents of $xyz
70 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
71 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
72 .sp
73 The \eQ...\eE sequence is recognized both inside and outside character classes.
74 .P
75 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
76 constructions. However, there is support for recursive patterns. This is not
77 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
78 feature allows an external function to be called during pattern matching. See
79 the
80 .\" HREF
81 \fBpcrecallout\fP
82 .\"
83 documentation for details.
84 .P
85 10. Subpatterns that are called recursively or as "subroutines" are always
86 treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
87 is a discussion of an example that explains this in more detail in the
88 .\" HTML <a href="pcrepattern.html#recursiondifference">
89 .\" </a>
90 section on recursion differences from Perl
91 .\"
92 in the
93 .\" HREF
94 \fBpcrepattern\fP
95 .\"
96 page.
97 .P
98 11. There are some differences that are concerned with the settings of captured
99 strings when part of a pattern is repeated. For example, matching "aba" against
100 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
101 .P
102 12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
103 names is not as general as Perl's. This is a consequence of the fact the PCRE
104 works internally just with numbers, using an external table to translate
105 between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
106 where the two capturing parentheses have the same number but different names,
107 is not supported, and causes an error at compile time. If it were allowed, it
108 would not be possible to distinguish which parentheses matched, because both
109 names map to capturing subpattern number 1. To avoid this confusing situation,
110 an error is given at compile time.
111 .P
112 13. Perl recognizes comments in some places that PCRE doesn't, for example,
113 between the ( and ? at the start of a subpattern.
114 .P
115 14. PCRE provides some extensions to the Perl regular expression facilities.
116 Perl 5.10 includes new features that are not in earlier versions of Perl, some
117 of which (such as named parentheses) have been in PCRE for some time. This list
118 is with respect to Perl 5.10:
119 .sp
120 (a) Although lookbehind assertions in PCRE must match fixed length strings,
121 each alternative branch of a lookbehind assertion can match a different length
122 of string. Perl requires them all to have the same length.
123 .sp
124 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
125 meta-character matches only at the very end of the string.
126 .sp
127 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
128 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
129 (Perl can be made to issue a warning.)
130 .sp
131 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
132 inverted, that is, by default they are not greedy, but if followed by a
133 question mark they are.
134 .sp
135 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
136 only at the first matching position in the subject string.
137 .sp
139 PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
140 .sp
141 (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
142 by the PCRE_BSR_ANYCRLF option.
143 .sp
144 (h) The callout facility is PCRE-specific.
145 .sp
146 (i) The partial matching facility is PCRE-specific.
147 .sp
148 (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
149 different hosts that have the other endianness.
150 .sp
151 (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
152 different way and is not Perl-compatible.
153 .sp
154 (l) PCRE recognizes some special sequences such as (*CR) at the start of
155 a pattern that set overall options that cannot be changed within the pattern.
156 .
157 .
159 .rs
160 .sp
161 .nf
162 Philip Hazel
163 University Computing Service
164 Cambridge CB2 3QH, England.
165 .fi
166 .
167 .
169 .rs
170 .sp
171 .nf
172 Last updated: 23 July 2011
173 Copyright (c) 1997-2011 University of Cambridge.
174 .fi


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