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1 nigel 79 .TH PCRECOMPAT 3
2 nigel 63 .SH NAME
3     PCRE - Perl-compatible regular expressions
4 nigel 75 .SH "DIFFERENCES BETWEEN PCRE AND PERL"
5 nigel 63 .rs
6     .sp
7     This document describes the differences in the ways that PCRE and Perl handle
8 nigel 93 regular expressions. The differences described here are mainly with respect to
9     Perl 5.8, though PCRE version 7.0 contains some features that are expected to
10     be in the forthcoming Perl 5.10.
11 nigel 75 .P
12 nigel 87 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 nigel 73 .\" HTML <a href="pcre.html#utf8support">
15     .\" </a>
16     section on UTF-8 support
17     .\"
18     in the main
19     .\" HREF
20 nigel 75 \fBpcre\fP
21 nigel 73 .\"
22     page.
23 nigel 75 .P
24 nigel 73 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
25 nigel 63 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 nigel 75 .P
29 nigel 73 3. Capturing subpatterns that occur inside negative lookahead assertions are
30 nigel 63 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 nigel 75 .P
35 nigel 73 4. Though binary zero characters are supported in the subject string, they are
36 nigel 63 not allowed in a pattern string because it is passed as a normal C string,
37 nigel 75 terminated by zero. The escape sequence \e0 can be used in the pattern to
38 nigel 63 represent a binary zero.
39 nigel 75 .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 nigel 87 Lu and Nd, script names such as Greek or Han, and the derived properties Any
49     and L&.
50 nigel 75 .P
51     7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
52 nigel 63 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 nigel 75 .sp
57 nigel 63 Pattern PCRE matches Perl matches
58 nigel 75 .sp
59     .\" JOIN
60     \eQabc$xyz\eE abc$xyz abc followed by the
61 nigel 63 contents of $xyz
62 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
63     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
64     .sp
65     The \eQ...\eE sequence is recognized both inside and outside character classes.
66     .P
67 nigel 93 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
68     constructions. However, there is support for recursive patterns. This is not
69     available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
70     feature allows an external function to be called during pattern matching. See
71     the
72 nigel 75 .\" HREF
73     \fBpcrecallout\fP
74     .\"
75     documentation for details.
76     .P
77 nigel 93 9. Subpatterns that are called recursively or as "subroutines" are always
78     treated as atomic groups in PCRE. This is like Python, but unlike Perl.
79     .P
80     10. There are some differences that are concerned with the settings of captured
81 nigel 63 strings when part of a pattern is repeated. For example, matching "aba" against
82     the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
83 nigel 75 .P
84 nigel 93 11. PCRE provides some extensions to the Perl regular expression facilities.
85     Perl 5.10 will include new features that are not in earlier versions, some of
86     which (such as named parentheses) have been in PCRE for some time. This list is
87     with respect to Perl 5.10:
88 nigel 75 .sp
89 nigel 63 (a) Although lookbehind assertions must match fixed length strings, each
90     alternative branch of a lookbehind assertion can match a different length of
91     string. Perl requires them all to have the same length.
92 nigel 75 .sp
93 nigel 63 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
94     meta-character matches only at the very end of the string.
95 nigel 75 .sp
96 nigel 63 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
97 nigel 91 meaning is faulted. Otherwise, like Perl, the backslash is ignored. (Perl can
98     be made to issue a warning.)
99 nigel 75 .sp
100 nigel 63 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
101     inverted, that is, by default they are not greedy, but if followed by a
102     question mark they are.
103 nigel 75 .sp
104     (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
105     only at the first matching position in the subject string.
106     .sp
107 nigel 63 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
108 nigel 75 options for \fBpcre_exec()\fP have no Perl equivalents.
109     .sp
110 nigel 93 (g) The callout facility is PCRE-specific.
111 nigel 75 .sp
112 nigel 93 (h) The partial matching facility is PCRE-specific.
113 nigel 75 .sp
114 nigel 93 (i) Patterns compiled by PCRE can be saved and re-used at a later time, even on
115 nigel 75 different hosts that have the other endianness.
116 nigel 77 .sp
117 nigel 93 (j) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
118 nigel 77 different way and is not Perl-compatible.
119 nigel 75 .P
120 nigel 63 .in 0
121 nigel 93 Last updated: 28 November 2006
122 nigel 63 .br
123 nigel 87 Copyright (c) 1997-2006 University of Cambridge.

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