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Contents of /code/trunk/doc/pcrecompat.3

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Load pcre-6.0 into code/trunk.
1 .TH PCRE 3
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 5.8.
10 .P
11 1. PCRE does not have full UTF-8 support. Details of what it does have are
12 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 lookahead assertions. Perl permits
24 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 .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. 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 Lu and Nd.
48 .P
49 7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
50 between are treated as literals. This is slightly different from Perl in that $
51 and @ are also handled as literals inside the quotes. In Perl, they cause
52 variable interpolation (but of course PCRE does not have variables). Note the
53 following examples:
54 .sp
55 Pattern PCRE matches Perl matches
56 .sp
57 .\" JOIN
58 \eQabc$xyz\eE abc$xyz abc followed by the
59 contents of $xyz
60 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
61 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
62 .sp
63 The \eQ...\eE sequence is recognized both inside and outside character classes.
64 .P
65 8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
66 constructions. However, there is support for recursive patterns using the
67 non-Perl items (?R), (?number), and (?P>name). Also, the PCRE "callout" feature
68 allows an external function to be called during pattern matching. See the
69 .\" HREF
70 \fBpcrecallout\fP
71 .\"
72 documentation for details.
73 .P
74 9. There are some differences that are concerned with the settings of captured
75 strings when part of a pattern is repeated. For example, matching "aba" against
76 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
77 .P
78 10. PCRE provides some extensions to the Perl regular expression facilities:
79 .sp
80 (a) Although lookbehind assertions must match fixed length strings, each
81 alternative branch of a lookbehind assertion can match a different length of
82 string. Perl requires them all to have the same length.
83 .sp
84 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
85 meta-character matches only at the very end of the string.
86 .sp
87 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
88 meaning is faulted.
89 .sp
90 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
91 inverted, that is, by default they are not greedy, but if followed by a
92 question mark they are.
93 .sp
94 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
95 only at the first matching position in the subject string.
96 .sp
98 options for \fBpcre_exec()\fP have no Perl equivalents.
99 .sp
100 (g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern
101 matching (Perl can do this using the (?p{code}) construct, which PCRE cannot
102 support.)
103 .sp
104 (h) PCRE supports named capturing substrings, using the Python syntax.
105 .sp
106 (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java
107 package.
108 .sp
109 (j) The (R) condition, for testing recursion, is a PCRE extension.
110 .sp
111 (k) The callout facility is PCRE-specific.
112 .sp
113 (l) The partial matching facility is PCRE-specific.
114 .sp
115 (m) Patterns compiled by PCRE can be saved and re-used at a later time, even on
116 different hosts that have the other endianness.
117 .sp
118 (n) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
119 different way and is not Perl-compatible.
120 .P
121 .in 0
122 Last updated: 28 February 2005
123 .br
124 Copyright (c) 1997-2005 University of Cambridge.

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