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3  <title>pcrecompat specification</title>  <title>pcrecompat specification</title>
4  </head>  </head>
5  <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">  <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6  This HTML document has been generated automatically from the original man page.  <h1>pcrecompat man page</h1>
7  If there is any nonsense in it, please consult the man page, in case the  <p>
8  conversion went wrong.<br>  Return to the <a href="index.html">PCRE index page</a>.
9  <ul>  </p>
10  <li><a name="TOC1" href="#SEC1">DIFFERENCES FROM PERL</a>  <p>
11  </ul>  This page is part of the PCRE HTML documentation. It was generated automatically
12  <br><a name="SEC1" href="#TOC1">DIFFERENCES FROM PERL</a><br>  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    <br>
15    <br><b>
16    DIFFERENCES BETWEEN PCRE AND PERL
17    </b><br>
18  <P>  <P>
19  This document describes the differences in the ways that PCRE and Perl handle  This document describes the differences in the ways that PCRE and Perl handle
20  regular expressions. The differences described here are with respect to Perl  regular expressions. The differences described here are mainly with respect to
21  5.8.  Perl 5.8, though PCRE version 7.0 contains some features that are expected to
22    be in the forthcoming Perl 5.10.
23    </P>
24    <P>
25    1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
26    it does have are given in the
27    <a href="pcre.html#utf8support">section on UTF-8 support</a>
28    in the main
29    <a href="pcre.html"><b>pcre</b></a>
30    page.
31  </P>  </P>
32  <P>  <P>
33  1. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits  2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
34  them, but they do not mean what you might think. For example, (?!a){3} does  them, but they do not mean what you might think. For example, (?!a){3} does
35  not assert that the next three characters are not "a". It just asserts that the  not assert that the next three characters are not "a". It just asserts that the
36  next character is not "a" three times.  next character is not "a" three times.
37  </P>  </P>
38  <P>  <P>
39  2. Capturing subpatterns that occur inside negative lookahead assertions are  3. Capturing subpatterns that occur inside negative lookahead assertions are
40  counted, but their entries in the offsets vector are never set. Perl sets its  counted, but their entries in the offsets vector are never set. Perl sets its
41  numerical variables from any such patterns that are matched before the  numerical variables from any such patterns that are matched before the
42  assertion fails to match something (thereby succeeding), but only if the  assertion fails to match something (thereby succeeding), but only if the
43  negative lookahead assertion contains just one branch.  negative lookahead assertion contains just one branch.
44  </P>  </P>
45  <P>  <P>
46  3. Though binary zero characters are supported in the subject string, they are  4. Though binary zero characters are supported in the subject string, they are
47  not allowed in a pattern string because it is passed as a normal C string,  not allowed in a pattern string because it is passed as a normal C string,
48  terminated by zero. The escape sequence "\0" can be used in the pattern to  terminated by zero. The escape sequence \0 can be used in the pattern to
49  represent a binary zero.  represent a binary zero.
50  </P>  </P>
51  <P>  <P>
52  4. The following Perl escape sequences are not supported: \l, \u, \L,  5. The following Perl escape sequences are not supported: \l, \u, \L,
53  \U, \P, \p, \N, and \X. In fact these are implemented by Perl's general  \U, and \N. In fact these are implemented by Perl's general string-handling
54  string-handling and are not part of its pattern matching engine. If any of  and are not part of its pattern matching engine. If any of these are
55  these are encountered by PCRE, an error is generated.  encountered by PCRE, an error is generated.
56  </P>  </P>
57  <P>  <P>
58  5. PCRE does support the \Q...\E escape for quoting substrings. Characters in  6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
59    built with Unicode character property support. The properties that can be
60    tested with \p and \P are limited to the general category properties such as
61    Lu and Nd, script names such as Greek or Han, and the derived properties Any
62    and L&.
63    </P>
64    <P>
65    7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
66  between are treated as literals. This is slightly different from Perl in that $  between are treated as literals. This is slightly different from Perl in that $
67  and @ are also handled as literals inside the quotes. In Perl, they cause  and @ are also handled as literals inside the quotes. In Perl, they cause
68  variable interpolation (but of course PCRE does not have variables). Note the  variable interpolation (but of course PCRE does not have variables). Note the
69  following examples:  following examples:
 </P>  
 <P>  
70  <pre>  <pre>
71      Pattern            PCRE matches      Perl matches      Pattern            PCRE matches      Perl matches
72  </PRE>  
73  </P>      \Qabc$xyz\E        abc$xyz           abc followed by the contents of $xyz
 <P>  
 <pre>  
     \Qabc$xyz\E        abc$xyz           abc followed by the  
                                            contents of $xyz  
74      \Qabc\$xyz\E       abc\$xyz          abc\$xyz      \Qabc\$xyz\E       abc\$xyz          abc\$xyz
75      \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz      \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
76  </PRE>  </pre>
77    The \Q...\E sequence is recognized both inside and outside character classes.
78  </P>  </P>
79  <P>  <P>
80  In PCRE, the \Q...\E mechanism is not recognized inside a character class.  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
81    constructions. However, there is support for recursive patterns. This is not
82    available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
83    feature allows an external function to be called during pattern matching. See
84    the
85    <a href="pcrecallout.html"><b>pcrecallout</b></a>
86    documentation for details.
87  </P>  </P>
88  <P>  <P>
89  8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})  9. Subpatterns that are called recursively or as "subroutines" are always
90  constructions. However, there is some experimental support for recursive  treated as atomic groups in PCRE. This is like Python, but unlike Perl.
 patterns using the non-Perl items (?R), (?number) and (?P&#62;name). Also, the PCRE  
 "callout" feature allows an external function to be called during pattern  
 matching.  
91  </P>  </P>
92  <P>  <P>
93  9. There are some differences that are concerned with the settings of captured  10. There are some differences that are concerned with the settings of captured
94  strings when part of a pattern is repeated. For example, matching "aba" against  strings when part of a pattern is repeated. For example, matching "aba" against
95  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
96  </P>  </P>
97  <P>  <P>
98  10. PCRE provides some extensions to the Perl regular expression facilities:  11. PCRE provides some extensions to the Perl regular expression facilities.
99  </P>  Perl 5.10 will include new features that are not in earlier versions, some of
100  <P>  which (such as named parentheses) have been in PCRE for some time. This list is
101    with respect to Perl 5.10:
102    <br>
103    <br>
104  (a) Although lookbehind assertions must match fixed length strings, each  (a) Although lookbehind assertions must match fixed length strings, each
105  alternative branch of a lookbehind assertion can match a different length of  alternative branch of a lookbehind assertion can match a different length of
106  string. Perl requires them all to have the same length.  string. Perl requires them all to have the same length.
107  </P>  <br>
108  <P>  <br>
109  (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $  (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
110  meta-character matches only at the very end of the string.  meta-character matches only at the very end of the string.
111  </P>  <br>
112  <P>  <br>
113  &copy; If PCRE_EXTRA is set, a backslash followed by a letter with no special  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
114  meaning is faulted.  meaning is faulted. Otherwise, like Perl, the backslash is ignored. (Perl can
115  </P>  be made to issue a warning.)
116  <P>  <br>
117    <br>
118  (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is  (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
119  inverted, that is, by default they are not greedy, but if followed by a  inverted, that is, by default they are not greedy, but if followed by a
120  question mark they are.  question mark they are.
121  </P>  <br>
122  <P>  <br>
123  (e) PCRE_ANCHORED can be used to force a pattern to be tried only at the first  (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
124  matching position in the subject string.  only at the first matching position in the subject string.
125  </P>  <br>
126  <P>  <br>
127  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
128  options for <b>pcre_exec()</b> have no Perl equivalents.  options for <b>pcre_exec()</b> have no Perl equivalents.
129    <br>
130    <br>
131    (g) The callout facility is PCRE-specific.
132    <br>
133    <br>
134    (h) The partial matching facility is PCRE-specific.
135    <br>
136    <br>
137    (i) Patterns compiled by PCRE can be saved and re-used at a later time, even on
138    different hosts that have the other endianness.
139    <br>
140    <br>
141    (j) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
142    different way and is not Perl-compatible.
143  </P>  </P>
144  <P>  <P>
145  (g) The (?R), (?number), and (?P&#62;name) constructs allows for recursive pattern  Last updated: 28 November 2006
 matching (Perl can do this using the (?p{code}) construct, which PCRE cannot  
 support.)  
 </P>  
 <P>  
 (h) PCRE supports named capturing substrings, using the Python syntax.  
 </P>  
 <P>  
 (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java  
 package.  
 </P>  
 <P>  
 (j) The (R) condition, for testing recursion, is a PCRE extension.  
 </P>  
 <P>  
 (k) The callout facility is PCRE-specific.  
 </P>  
 <P>  
 Last updated: 03 February 2003  
146  <br>  <br>
147  Copyright &copy; 1997-2003 University of Cambridge.  Copyright &copy; 1997-2006 University of Cambridge.
148    <p>
149    Return to the <a href="index.html">PCRE index page</a>.
150    </p>

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