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1 <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 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 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 3. Capturing subpatterns that occur inside negative lookahead assertions are
34 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 4. Though binary zero characters are supported in the subject string, they are
41 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 5. The following Perl escape sequences are not supported: \l, \u, \L,
47 \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 6. PCRE does support the \Q...\E escape for quoting substrings. Characters in
53 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$xyz
69 </PRE>
70 </P>
71 <P>
72 The \Q...\E sequence is recognized both inside and outside character classes.
73 </P>
74 <P>
75 7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
76 constructions. However, there is some experimental support for recursive
77 patterns using the non-Perl items (?R), (?number) and (?P&#62;name). Also, the PCRE
78 "callout" feature allows an external function to be called during pattern
79 matching.
80 </P>
81 <P>
82 8. There are some differences that are concerned with the settings of captured
83 strings when part of a pattern is repeated. For example, matching "aba" against
84 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
85 </P>
86 <P>
87 9. PCRE provides some extensions to the Perl regular expression facilities:
88 </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 Last updated: 09 December 2003
135 <br>
136 Copyright &copy; 1997-2003 University of Cambridge.

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