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Wed Sep 16 10:56:40 2009 UTC (4 years, 7 months ago) by ph10
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Minor doc fix; update tests by adding comments to each file, and generally 
tidying.

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 ph10 450 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
50     Perl documentation says "Because Perl hides the need for the user to understand
51     the internal representation of Unicode characters, there is no need to
52     implement the somewhat messy concept of surrogates."
53 nigel 75 .P
54     7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
55 nigel 63 between are treated as literals. This is slightly different from Perl in that $
56     and @ are also handled as literals inside the quotes. In Perl, they cause
57     variable interpolation (but of course PCRE does not have variables). Note the
58     following examples:
59 nigel 75 .sp
60 nigel 63 Pattern PCRE matches Perl matches
61 nigel 75 .sp
62     .\" JOIN
63     \eQabc$xyz\eE abc$xyz abc followed by the
64 nigel 63 contents of $xyz
65 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
66     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
67     .sp
68     The \eQ...\eE sequence is recognized both inside and outside character classes.
69     .P
70 nigel 93 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
71     constructions. However, there is support for recursive patterns. This is not
72     available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
73     feature allows an external function to be called during pattern matching. See
74     the
75 nigel 75 .\" HREF
76     \fBpcrecallout\fP
77     .\"
78     documentation for details.
79     .P
80 nigel 93 9. Subpatterns that are called recursively or as "subroutines" are always
81     treated as atomic groups in PCRE. This is like Python, but unlike Perl.
82     .P
83     10. There are some differences that are concerned with the settings of captured
84 nigel 63 strings when part of a pattern is repeated. For example, matching "aba" against
85     the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
86 nigel 75 .P
87 ph10 210 11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
88     (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
89 ph10 447 argument. PCRE does not support (*MARK).
90 ph10 210 .P
91     12. PCRE provides some extensions to the Perl regular expression facilities.
92 nigel 93 Perl 5.10 will include new features that are not in earlier versions, some of
93     which (such as named parentheses) have been in PCRE for some time. This list is
94     with respect to Perl 5.10:
95 nigel 75 .sp
96 nigel 63 (a) Although lookbehind assertions must match fixed length strings, each
97     alternative branch of a lookbehind assertion can match a different length of
98     string. Perl requires them all to have the same length.
99 nigel 75 .sp
100 nigel 63 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
101     meta-character matches only at the very end of the string.
102 nigel 75 .sp
103 nigel 63 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
104 ph10 181 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
105     (Perl can be made to issue a warning.)
106 nigel 75 .sp
107 nigel 63 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
108     inverted, that is, by default they are not greedy, but if followed by a
109     question mark they are.
110 nigel 75 .sp
111     (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
112     only at the first matching position in the subject string.
113     .sp
114 ph10 442 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
115     PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
116 nigel 75 .sp
117 ph10 231 (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
118     by the PCRE_BSR_ANYCRLF option.
119 nigel 75 .sp
120 ph10 231 (h) The callout facility is PCRE-specific.
121 nigel 75 .sp
122 ph10 231 (i) The partial matching facility is PCRE-specific.
123     .sp
124     (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
125 nigel 75 different hosts that have the other endianness.
126 nigel 77 .sp
127 ph10 231 (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
128 nigel 77 different way and is not Perl-compatible.
129 ph10 231 .sp
130     (l) PCRE recognizes some special sequences such as (*CR) at the start of
131     a pattern that set overall options that cannot be changed within the pattern.
132 ph10 99 .
133     .
134     .SH AUTHOR
135     .rs
136     .sp
137     .nf
138     Philip Hazel
139     University Computing Service
140     Cambridge CB2 3QH, England.
141     .fi
142     .
143     .
144     .SH REVISION
145     .rs
146     .sp
147     .nf
148 ph10 450 Last updated: 16 September 2009
149 ph10 426 Copyright (c) 1997-2009 University of Cambridge.
150 ph10 99 .fi

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