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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     regular expressions. The differences described here are with respect to Perl
9     5.8.
10 nigel 75 .P
11 nigel 87 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 nigel 73 .\" HTML <a href="pcre.html#utf8support">
14     .\" </a>
15     section on UTF-8 support
16     .\"
17     in the main
18     .\" HREF
19 nigel 75 \fBpcre\fP
20 nigel 73 .\"
21     page.
22 nigel 75 .P
23 nigel 73 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
24 nigel 63 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 nigel 75 .P
28 nigel 73 3. Capturing subpatterns that occur inside negative lookahead assertions are
29 nigel 63 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 nigel 75 .P
34 nigel 73 4. Though binary zero characters are supported in the subject string, they are
35 nigel 63 not allowed in a pattern string because it is passed as a normal C string,
36 nigel 75 terminated by zero. The escape sequence \e0 can be used in the pattern to
37 nigel 63 represent a binary zero.
38 nigel 75 .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 nigel 87 Lu and Nd, script names such as Greek or Han, and the derived properties Any
48     and L&.
49 nigel 75 .P
50     7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
51 nigel 63 between are treated as literals. This is slightly different from Perl in that $
52     and @ are also handled as literals inside the quotes. In Perl, they cause
53     variable interpolation (but of course PCRE does not have variables). Note the
54     following examples:
55 nigel 75 .sp
56 nigel 63 Pattern PCRE matches Perl matches
57 nigel 75 .sp
58     .\" JOIN
59     \eQabc$xyz\eE abc$xyz abc followed by the
60 nigel 63 contents of $xyz
61 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
62     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
63     .sp
64     The \eQ...\eE sequence is recognized both inside and outside character classes.
65     .P
66     8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
67     constructions. However, there is support for recursive patterns using the
68     non-Perl items (?R), (?number), and (?P>name). Also, the PCRE "callout" feature
69     allows an external function to be called during pattern matching. See the
70     .\" HREF
71     \fBpcrecallout\fP
72     .\"
73     documentation for details.
74     .P
75     9. There are some differences that are concerned with the settings of captured
76 nigel 63 strings when part of a pattern is repeated. For example, matching "aba" against
77     the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
78 nigel 75 .P
79     10. PCRE provides some extensions to the Perl regular expression facilities:
80     .sp
81 nigel 63 (a) Although lookbehind assertions must match fixed length strings, each
82     alternative branch of a lookbehind assertion can match a different length of
83     string. Perl requires them all to have the same length.
84 nigel 75 .sp
85 nigel 63 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
86     meta-character matches only at the very end of the string.
87 nigel 75 .sp
88 nigel 63 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
89     meaning is faulted.
90 nigel 75 .sp
91 nigel 63 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
92     inverted, that is, by default they are not greedy, but if followed by a
93     question mark they are.
94 nigel 75 .sp
95     (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
96     only at the first matching position in the subject string.
97     .sp
98 nigel 63 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
99 nigel 75 options for \fBpcre_exec()\fP have no Perl equivalents.
100     .sp
101 nigel 63 (g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern
102     matching (Perl can do this using the (?p{code}) construct, which PCRE cannot
103     support.)
104 nigel 75 .sp
105 nigel 63 (h) PCRE supports named capturing substrings, using the Python syntax.
106 nigel 75 .sp
107 nigel 63 (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java
108     package.
109 nigel 75 .sp
110 nigel 63 (j) The (R) condition, for testing recursion, is a PCRE extension.
111 nigel 75 .sp
112 nigel 63 (k) The callout facility is PCRE-specific.
113 nigel 75 .sp
114     (l) The partial matching facility is PCRE-specific.
115     .sp
116     (m) Patterns compiled by PCRE can be saved and re-used at a later time, even on
117     different hosts that have the other endianness.
118 nigel 77 .sp
119     (n) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
120     different way and is not Perl-compatible.
121 nigel 75 .P
122 nigel 63 .in 0
123 nigel 87 Last updated: 24 January 2006
124 nigel 63 .br
125 nigel 87 Copyright (c) 1997-2006 University of Cambridge.

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