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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 nigel 75 <h1>pcrecompat man page</h1>
7     <p>
8     Return to the <a href="index.html">PCRE index page</a>.
9     </p>
10 ph10 111 <p>
11 nigel 75 This page is part of the PCRE HTML documentation. It was generated automatically
12     from the original man page. If there is any nonsense in it, please consult the
13     man page, in case the conversion went wrong.
14 ph10 111 <br>
15 nigel 75 <br><b>
16     DIFFERENCES BETWEEN PCRE AND PERL
17     </b><br>
18 nigel 63 <P>
19     This document describes the differences in the ways that PCRE and Perl handle
20 ph10 461 regular expressions. The differences described here are with respect to Perl
21 ph10 567 versions 5.10 and above.
22 nigel 63 </P>
23     <P>
24 nigel 87 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
25     it does have are given in the
26 nigel 73 <a href="pcre.html#utf8support">section on UTF-8 support</a>
27     in the main
28     <a href="pcre.html"><b>pcre</b></a>
29     page.
30     </P>
31     <P>
32     2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
33 nigel 63 them, but they do not mean what you might think. For example, (?!a){3} does
34     not assert that the next three characters are not "a". It just asserts that the
35     next character is not "a" three times.
36     </P>
37     <P>
38 nigel 73 3. Capturing subpatterns that occur inside negative lookahead assertions are
39 nigel 63 counted, but their entries in the offsets vector are never set. Perl sets its
40     numerical variables from any such patterns that are matched before the
41     assertion fails to match something (thereby succeeding), but only if the
42     negative lookahead assertion contains just one branch.
43     </P>
44     <P>
45 nigel 73 4. Though binary zero characters are supported in the subject string, they are
46 nigel 63 not allowed in a pattern string because it is passed as a normal C string,
47 nigel 75 terminated by zero. The escape sequence \0 can be used in the pattern to
48 nigel 63 represent a binary zero.
49     </P>
50     <P>
51 nigel 73 5. The following Perl escape sequences are not supported: \l, \u, \L,
52 nigel 75 \U, and \N. In fact these are implemented by Perl's general string-handling
53     and are not part of its pattern matching engine. If any of these are
54     encountered by PCRE, an error is generated.
55 nigel 63 </P>
56     <P>
57 nigel 75 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
58     built with Unicode character property support. The properties that can be
59     tested with \p and \P are limited to the general category properties such as
60 nigel 87 Lu and Nd, script names such as Greek or Han, and the derived properties Any
61 ph10 453 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
62     Perl documentation says "Because Perl hides the need for the user to understand
63     the internal representation of Unicode characters, there is no need to
64     implement the somewhat messy concept of surrogates."
65 nigel 75 </P>
66     <P>
67     7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
68 nigel 63 between are treated as literals. This is slightly different from Perl in that $
69     and @ are also handled as literals inside the quotes. In Perl, they cause
70     variable interpolation (but of course PCRE does not have variables). Note the
71     following examples:
72     <pre>
73     Pattern PCRE matches Perl matches
74 nigel 75
75     \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
76 nigel 63 \Qabc\$xyz\E abc\$xyz abc\$xyz
77     \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
78 nigel 75 </pre>
79 nigel 73 The \Q...\E sequence is recognized both inside and outside character classes.
80 nigel 63 </P>
81     <P>
82 nigel 93 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
83     constructions. However, there is support for recursive patterns. This is not
84 ph10 453 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
85 nigel 93 feature allows an external function to be called during pattern matching. See
86     the
87 nigel 75 <a href="pcrecallout.html"><b>pcrecallout</b></a>
88     documentation for details.
89 nigel 63 </P>
90     <P>
91 nigel 93 9. Subpatterns that are called recursively or as "subroutines" are always
92 ph10 461 treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
93 ph10 453 is a discussion of an example that explains this in more detail in the
94     <a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
95     in the
96 ph10 461 <a href="pcrepattern.html"><b>pcrepattern</b></a>
97 ph10 453 page.
98 nigel 93 </P>
99     <P>
100     10. There are some differences that are concerned with the settings of captured
101 nigel 63 strings when part of a pattern is repeated. For example, matching "aba" against
102     the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
103     </P>
104     <P>
105 ph10 535 11. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
106 ph10 461 names is not as general as Perl's. This is a consequence of the fact the PCRE
107     works internally just with numbers, using an external table to translate
108     between numbers and names. In particular, a pattern such as (?|(?&#60;a&#62;A)|(?&#60;b)B),
109     where the two capturing parentheses have the same number but different names,
110     is not supported, and causes an error at compile time. If it were allowed, it
111     would not be possible to distinguish which parentheses matched, because both
112     names map to capturing subpattern number 1. To avoid this confusing situation,
113     an error is given at compile time.
114     </P>
115     <P>
116 ph10 579 12. Perl recognizes comments in some places that PCRE doesn't, for example,
117 ph10 567 between the ( and ? at the start of a subpattern.
118     </P>
119     <P>
120     13. PCRE provides some extensions to the Perl regular expression facilities.
121 ph10 461 Perl 5.10 includes new features that are not in earlier versions of Perl, some
122     of which (such as named parentheses) have been in PCRE for some time. This list
123     is with respect to Perl 5.10:
124 nigel 75 <br>
125     <br>
126 ph10 461 (a) Although lookbehind assertions in PCRE must match fixed length strings,
127     each alternative branch of a lookbehind assertion can match a different length
128     of string. Perl requires them all to have the same length.
129 nigel 75 <br>
130     <br>
131 nigel 63 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
132     meta-character matches only at the very end of the string.
133 nigel 75 <br>
134     <br>
135     (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
136 ph10 182 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
137     (Perl can be made to issue a warning.)
138 nigel 75 <br>
139     <br>
140 nigel 63 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
141     inverted, that is, by default they are not greedy, but if followed by a
142     question mark they are.
143 nigel 75 <br>
144     <br>
145     (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
146     only at the first matching position in the subject string.
147     <br>
148     <br>
149 ph10 453 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
150     PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
151 nigel 75 <br>
152     <br>
153 ph10 231 (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
154     by the PCRE_BSR_ANYCRLF option.
155 nigel 75 <br>
156     <br>
157 ph10 231 (h) The callout facility is PCRE-specific.
158 nigel 75 <br>
159     <br>
160 ph10 231 (i) The partial matching facility is PCRE-specific.
161     <br>
162     <br>
163     (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
164 nigel 75 different hosts that have the other endianness.
165 nigel 77 <br>
166     <br>
167 ph10 231 (k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
168 nigel 77 different way and is not Perl-compatible.
169 ph10 231 <br>
170     <br>
171     (l) PCRE recognizes some special sequences such as (*CR) at the start of
172     a pattern that set overall options that cannot be changed within the pattern.
173 nigel 63 </P>
174 ph10 99 <br><b>
175     AUTHOR
176     </b><br>
177 nigel 63 <P>
178 ph10 99 Philip Hazel
179 nigel 63 <br>
180 ph10 99 University Computing Service
181     <br>
182     Cambridge CB2 3QH, England.
183     <br>
184     </P>
185     <br><b>
186     REVISION
187     </b><br>
188     <P>
189 ph10 567 Last updated: 31 October 2010
190 ph10 99 <br>
191 ph10 535 Copyright &copy; 1997-2010 University of Cambridge.
192 ph10 99 <br>
193 nigel 75 <p>
194     Return to the <a href="index.html">PCRE index page</a>.
195     </p>

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