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Revision 442 - (hide annotations) (download)
Fri Sep 11 10:21:02 2009 UTC (5 years ago) by ph10
File size: 5980 byte(s)
Added PCRE_NOTEMPTY_ATSTART to fix /g bug when \K is present.

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 ph10 181 Perl 5.8, though PCRE versions 7.0 and later contain some features that are
10 ph10 426 in 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 ph10 210 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 ph10 211 argument. PCRE does not support (*MARK). If (*ACCEPT) is within capturing
87 ph10 210 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 nigel 93 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 nigel 75 .sp
94 nigel 63 (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 nigel 75 .sp
98 nigel 63 (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 nigel 75 .sp
101 nigel 63 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
102 ph10 181 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
103     (Perl can be made to issue a warning.)
104 nigel 75 .sp
105 nigel 63 (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 nigel 75 .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
112 ph10 442 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
113     PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
114 nigel 75 .sp
115 ph10 231 (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
116     by the PCRE_BSR_ANYCRLF option.
117 nigel 75 .sp
118 ph10 231 (h) The callout facility is PCRE-specific.
119 nigel 75 .sp
120 ph10 231 (i) The partial matching facility is PCRE-specific.
121     .sp
122     (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
123 nigel 75 different hosts that have the other endianness.
124 nigel 77 .sp
125 ph10 231 (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
126 nigel 77 different way and is not Perl-compatible.
127 ph10 231 .sp
128     (l) PCRE recognizes some special sequences such as (*CR) at the start of
129     a pattern that set overall options that cannot be changed within the pattern.
130 ph10 99 .
131     .
132     .SH AUTHOR
133     .rs
134     .sp
135     .nf
136     Philip Hazel
137     University Computing Service
138     Cambridge CB2 3QH, England.
139     .fi
140     .
141     .
142     .SH REVISION
143     .rs
144     .sp
145     .nf
146 ph10 442 Last updated: 11 September 2009
147 ph10 426 Copyright (c) 1997-2009 University of Cambridge.
148 ph10 99 .fi

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