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Revision 459 - (hide annotations) (download)
Sun Oct 4 09:21:39 2009 UTC (4 years, 10 months ago) by ph10
File size: 6961 byte(s)
Fix problems with conditional references to duplicate named subpatterns.

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 ph10 456 regular expressions. The differences described here are with respect to Perl
9     5.10.
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 ph10 450 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
49     Perl documentation says "Because Perl hides the need for the user to understand
50     the internal representation of Unicode characters, there is no need to
51     implement the somewhat messy concept of surrogates."
52 nigel 75 .P
53     7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
54 nigel 63 between are treated as literals. This is slightly different from Perl in that $
55     and @ are also handled as literals inside the quotes. In Perl, they cause
56     variable interpolation (but of course PCRE does not have variables). Note the
57     following examples:
58 nigel 75 .sp
59 nigel 63 Pattern PCRE matches Perl matches
60 nigel 75 .sp
61     .\" JOIN
62     \eQabc$xyz\eE abc$xyz abc followed by the
63 nigel 63 contents of $xyz
64 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
65     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
66     .sp
67     The \eQ...\eE sequence is recognized both inside and outside character classes.
68     .P
69 nigel 93 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
70     constructions. However, there is support for recursive patterns. This is not
71 ph10 453 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
72 nigel 93 feature allows an external function to be called during pattern matching. See
73     the
74 nigel 75 .\" HREF
75     \fBpcrecallout\fP
76     .\"
77     documentation for details.
78     .P
79 nigel 93 9. Subpatterns that are called recursively or as "subroutines" are always
80 ph10 453 treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
81     is a discussion of an example that explains this in more detail in the
82     .\" HTML <a href="pcrepattern.html#recursiondifference">
83     .\" </a>
84     section on recursion differences from Perl
85     .\"
86     in the
87     .\" HREF
88 ph10 456 \fBpcrepattern\fP
89 ph10 453 .\"
90     page.
91 nigel 93 .P
92     10. There are some differences that are concerned with the settings of captured
93 nigel 63 strings when part of a pattern is repeated. For example, matching "aba" against
94     the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
95 nigel 75 .P
96 ph10 210 11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
97     (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
98 ph10 447 argument. PCRE does not support (*MARK).
99 ph10 210 .P
100 ph10 456 12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
101 ph10 459 names is not as general as Perl's. This is a consequence of the fact the PCRE
102     works internally just with numbers, using an external table to translate
103     between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
104     where the two capturing parentheses have the same number but different names,
105     is not supported, and causes an error at compile time. If it were allowed, it
106     would not be possible to distinguish which parentheses matched, because both
107     names map to capturing subpattern number 1. To avoid this confusing situation,
108     an error is given at compile time.
109 ph10 456 .P
110     13. PCRE provides some extensions to the Perl regular expression facilities.
111     Perl 5.10 includes new features that are not in earlier versions of Perl, some
112     of which (such as named parentheses) have been in PCRE for some time. This list
113     is with respect to Perl 5.10:
114     .sp
115     (a) Although lookbehind assertions in PCRE must match fixed length strings,
116     each alternative branch of a lookbehind assertion can match a different length
117     of string. Perl requires them all to have the same length.
118     .sp
119 nigel 63 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
120     meta-character matches only at the very end of the string.
121 nigel 75 .sp
122 nigel 63 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
123 ph10 181 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
124     (Perl can be made to issue a warning.)
125 nigel 75 .sp
126 nigel 63 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
127     inverted, that is, by default they are not greedy, but if followed by a
128     question mark they are.
129 nigel 75 .sp
130     (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
131     only at the first matching position in the subject string.
132     .sp
133 ph10 442 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
134     PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
135 nigel 75 .sp
136 ph10 231 (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
137     by the PCRE_BSR_ANYCRLF option.
138 nigel 75 .sp
139 ph10 231 (h) The callout facility is PCRE-specific.
140 nigel 75 .sp
141 ph10 231 (i) The partial matching facility is PCRE-specific.
142     .sp
143     (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
144 nigel 75 different hosts that have the other endianness.
145 nigel 77 .sp
146 ph10 231 (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
147 nigel 77 different way and is not Perl-compatible.
148 ph10 231 .sp
149     (l) PCRE recognizes some special sequences such as (*CR) at the start of
150     a pattern that set overall options that cannot be changed within the pattern.
151 ph10 99 .
152     .
153     .SH AUTHOR
154     .rs
155     .sp
156     .nf
157     Philip Hazel
158     University Computing Service
159     Cambridge CB2 3QH, England.
160     .fi
161     .
162     .
163     .SH REVISION
164     .rs
165     .sp
166     .nf
167 ph10 459 Last updated: 04 October 2009
168 ph10 426 Copyright (c) 1997-2009 University of Cambridge.
169 ph10 99 .fi

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