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1 nigel 63 <html>
2     <head>
3     <title>pcrecompat specification</title>
4     </head>
5     <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6     This HTML document has been generated automatically from the original man page.
7     If there is any nonsense in it, please consult the man page, in case the
8     conversion went wrong.<br>
9     <ul>
10     <li><a name="TOC1" href="#SEC1">DIFFERENCES FROM PERL</a>
11     </ul>
12     <br><a name="SEC1" href="#TOC1">DIFFERENCES FROM PERL</a><br>
13     <P>
14     This document describes the differences in the ways that PCRE and Perl handle
15     regular expressions. The differences described here are with respect to Perl
16     5.8.
17     </P>
18     <P>
19 nigel 73 1. PCRE does not have full UTF-8 support. Details of what it does have are
20     given in the
21     <a href="pcre.html#utf8support">section on UTF-8 support</a>
22     in the main
23     <a href="pcre.html"><b>pcre</b></a>
24     page.
25     </P>
26     <P>
27     2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
28 nigel 63 them, but they do not mean what you might think. For example, (?!a){3} does
29     not assert that the next three characters are not "a". It just asserts that the
30     next character is not "a" three times.
31     </P>
32     <P>
33 nigel 73 3. Capturing subpatterns that occur inside negative lookahead assertions are
34 nigel 63 counted, but their entries in the offsets vector are never set. Perl sets its
35     numerical variables from any such patterns that are matched before the
36     assertion fails to match something (thereby succeeding), but only if the
37     negative lookahead assertion contains just one branch.
38     </P>
39     <P>
40 nigel 73 4. Though binary zero characters are supported in the subject string, they are
41 nigel 63 not allowed in a pattern string because it is passed as a normal C string,
42     terminated by zero. The escape sequence "\0" can be used in the pattern to
43     represent a binary zero.
44     </P>
45     <P>
46 nigel 73 5. The following Perl escape sequences are not supported: \l, \u, \L,
47 nigel 63 \U, \P, \p, \N, and \X. In fact these are implemented by Perl's general
48     string-handling and are not part of its pattern matching engine. If any of
49     these are encountered by PCRE, an error is generated.
50     </P>
51     <P>
52 nigel 73 6. PCRE does support the \Q...\E escape for quoting substrings. Characters in
53 nigel 63 between are treated as literals. This is slightly different from Perl in that $
54     and @ are also handled as literals inside the quotes. In Perl, they cause
55     variable interpolation (but of course PCRE does not have variables). Note the
56     following examples:
57     </P>
58     <P>
59     <pre>
60     Pattern PCRE matches Perl matches
61     </PRE>
62     </P>
63     <P>
64     <pre>
65     \Qabc$xyz\E abc$xyz abc followed by the
66     contents of $xyz
67     \Qabc\$xyz\E abc\$xyz abc\$xyz
68     \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
69     </PRE>
70     </P>
71     <P>
72 nigel 73 The \Q...\E sequence is recognized both inside and outside character classes.
73 nigel 63 </P>
74     <P>
75 nigel 73 7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
76 nigel 63 constructions. However, there is some experimental support for recursive
77     patterns using the non-Perl items (?R), (?number) and (?P&#62;name). Also, the PCRE
78     "callout" feature allows an external function to be called during pattern
79     matching.
80     </P>
81     <P>
82 nigel 73 8. There are some differences that are concerned with the settings of captured
83 nigel 63 strings when part of a pattern is repeated. For example, matching "aba" against
84     the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
85     </P>
86     <P>
87 nigel 73 9. PCRE provides some extensions to the Perl regular expression facilities:
88 nigel 63 </P>
89     <P>
90     (a) Although lookbehind assertions must match fixed length strings, each
91     alternative branch of a lookbehind assertion can match a different length of
92     string. Perl requires them all to have the same length.
93     </P>
94     <P>
95     (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
96     meta-character matches only at the very end of the string.
97     </P>
98     <P>
99     &copy; If PCRE_EXTRA is set, a backslash followed by a letter with no special
100     meaning is faulted.
101     </P>
102     <P>
103     (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
104     inverted, that is, by default they are not greedy, but if followed by a
105     question mark they are.
106     </P>
107     <P>
108     (e) PCRE_ANCHORED can be used to force a pattern to be tried only at the first
109     matching position in the subject string.
110     </P>
111     <P>
113     options for <b>pcre_exec()</b> have no Perl equivalents.
114     </P>
115     <P>
116     (g) The (?R), (?number), and (?P&#62;name) constructs allows for recursive pattern
117     matching (Perl can do this using the (?p{code}) construct, which PCRE cannot
118     support.)
119     </P>
120     <P>
121     (h) PCRE supports named capturing substrings, using the Python syntax.
122     </P>
123     <P>
124     (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java
125     package.
126     </P>
127     <P>
128     (j) The (R) condition, for testing recursion, is a PCRE extension.
129     </P>
130     <P>
131     (k) The callout facility is PCRE-specific.
132     </P>
133     <P>
134 nigel 73 Last updated: 09 December 2003
135 nigel 63 <br>
136     Copyright &copy; 1997-2003 University of Cambridge.

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