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revision 231 by ph10, Tue Sep 11 11:15:33 2007 UTC revision 836 by ph10, Wed Dec 28 17:16:11 2011 UTC
# Line 17  DIFFERENCES BETWEEN PCRE AND PERL Line 17  DIFFERENCES BETWEEN PCRE AND PERL
17  </b><br>  </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 mainly with respect to  regular expressions. The differences described here are with respect to Perl
21  Perl 5.8, though PCRE versions 7.0 and later contain some features that are  versions 5.10 and above.
 expected to be in the forthcoming Perl 5.10.  
22  </P>  </P>
23  <P>  <P>
24  1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what  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  it does have are given in the
26  <a href="pcre.html#utf8support">section on UTF-8 support</a>  <a href="pcreunicode.html"><b>pcreunicode</b></a>
 in the main  
 <a href="pcre.html"><b>pcre</b></a>  
27  page.  page.
28  </P>  </P>
29  <P>  <P>
30  2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits  2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
31  them, but they do not mean what you might think. For example, (?!a){3} does  not mean what you might think. For example, (?!a){3} does not assert that the
32  not assert that the next three characters are not "a". It just asserts that the  next three characters are not "a". It just asserts that the next character is
33  next character is not "a" three times.  not "a" three times (in principle: PCRE optimizes this to run the assertion
34    just once). Perl allows repeat quantifiers on other assertions such as \b, but
35    these do not seem to have any use.
36  </P>  </P>
37  <P>  <P>
38  3. Capturing subpatterns that occur inside negative lookahead assertions are  3. Capturing subpatterns that occur inside negative lookahead assertions are
# Line 50  represent a binary zero. Line 49  represent a binary zero.
49  </P>  </P>
50  <P>  <P>
51  5. The following Perl escape sequences are not supported: \l, \u, \L,  5. The following Perl escape sequences are not supported: \l, \u, \L,
52  \U, and \N. In fact these are implemented by Perl's general string-handling  \U, and \N when followed by a character name or Unicode value. (\N on its
53  and are not part of its pattern matching engine. If any of these are  own, matching a non-newline character, is supported.) In fact these are
54  encountered by PCRE, an error is generated.  implemented by Perl's general string-handling and are not part of its pattern
55    matching engine. If any of these are encountered by PCRE, an error is
56    generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
57    \U and \u are interpreted as JavaScript interprets them.
58  </P>  </P>
59  <P>  <P>
60  6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is  6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
61  built with Unicode character property support. The properties that can be  built with Unicode character property support. The properties that can be
62  tested with \p and \P are limited to the general category properties such as  tested with \p and \P are limited to the general category properties such as
63  Lu and Nd, script names such as Greek or Han, and the derived properties Any  Lu and Nd, script names such as Greek or Han, and the derived properties Any
64  and L&.  and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
65    Perl documentation says "Because Perl hides the need for the user to understand
66    the internal representation of Unicode characters, there is no need to
67    implement the somewhat messy concept of surrogates."
68  </P>  </P>
69  <P>  <P>
70  7. PCRE does support the \Q...\E escape for quoting substrings. Characters in  7. PCRE implements a simpler version of \X than Perl, which changed to make
71    \X match what Unicode calls an "extended grapheme cluster". This is more
72    complicated than an extended Unicode sequence, which is what PCRE matches.
73    </P>
74    <P>
75    8. PCRE does support the \Q...\E escape for quoting substrings. Characters in
76  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 $
77  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
78  variable interpolation (but of course PCRE does not have variables). Note the  variable interpolation (but of course PCRE does not have variables). Note the
# Line 77  following examples: Line 87  following examples:
87  The \Q...\E sequence is recognized both inside and outside character classes.  The \Q...\E sequence is recognized both inside and outside character classes.
88  </P>  </P>
89  <P>  <P>
90  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})  9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
91  constructions. However, there is support for recursive patterns. This is not  constructions. However, there is support for recursive patterns. This is not
92  available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"  available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
93  feature allows an external function to be called during pattern matching. See  feature allows an external function to be called during pattern matching. See
94  the  the
95  <a href="pcrecallout.html"><b>pcrecallout</b></a>  <a href="pcrecallout.html"><b>pcrecallout</b></a>
96  documentation for details.  documentation for details.
97  </P>  </P>
98  <P>  <P>
99  9. Subpatterns that are called recursively or as "subroutines" are always  10. Subpatterns that are called as subroutines (whether or not recursively) are
100  treated as atomic groups in PCRE. This is like Python, but unlike Perl.  always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
101    Captured values that are set outside a subroutine call can be reference from
102    inside in PCRE, but not in Perl. There is a discussion that explains these
103    differences in more detail in the
104    <a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
105    in the
106    <a href="pcrepattern.html"><b>pcrepattern</b></a>
107    page.
108  </P>  </P>
109  <P>  <P>
110  10. There are some differences that are concerned with the settings of captured  11. If (*THEN) is present in a group that is called as a subroutine, its action
111  strings when part of a pattern is repeated. For example, matching "aba" against  is limited to that group, even if the group does not contain any | characters.
 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".  
112  </P>  </P>
113  <P>  <P>
114  11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),  12. There are some differences that are concerned with the settings of captured
115  (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an  strings when part of a pattern is repeated. For example, matching "aba" against
116  argument. PCRE does not support (*MARK). If (*ACCEPT) is within capturing  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
 parentheses, PCRE does not set that capture group; this is different to Perl.  
117  </P>  </P>
118  <P>  <P>
119  12. PCRE provides some extensions to the Perl regular expression facilities.  13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
120  Perl 5.10 will include new features that are not in earlier versions, some of  names is not as general as Perl's. This is a consequence of the fact the PCRE
121  which (such as named parentheses) have been in PCRE for some time. This list is  works internally just with numbers, using an external table to translate
122  with respect to Perl 5.10:  between numbers and names. In particular, a pattern such as (?|(?&#60;a&#62;A)|(?&#60;b)B),
123  <br>  where the two capturing parentheses have the same number but different names,
124  <br>  is not supported, and causes an error at compile time. If it were allowed, it
125  (a) Although lookbehind assertions must match fixed length strings, each  would not be possible to distinguish which parentheses matched, because both
126  alternative branch of a lookbehind assertion can match a different length of  names map to capturing subpattern number 1. To avoid this confusing situation,
127  string. Perl requires them all to have the same length.  an error is given at compile time.
128    </P>
129    <P>
130    14. Perl recognizes comments in some places that PCRE does not, for example,
131    between the ( and ? at the start of a subpattern. If the /x modifier is set,
132    Perl allows whitespace between ( and ? but PCRE never does, even if the
133    PCRE_EXTENDED option is set.
134    </P>
135    <P>
136    15. PCRE provides some extensions to the Perl regular expression facilities.
137    Perl 5.10 includes new features that are not in earlier versions of Perl, some
138    of which (such as named parentheses) have been in PCRE for some time. This list
139    is with respect to Perl 5.10:
140    <br>
141    <br>
142    (a) Although lookbehind assertions in PCRE must match fixed length strings,
143    each alternative branch of a lookbehind assertion can match a different length
144    of string. Perl requires them all to have the same length.
145  <br>  <br>
146  <br>  <br>
147  (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 $
# Line 130  question mark they are. Line 162  question mark they are.
162  only at the first matching position in the subject string.  only at the first matching position in the subject string.
163  <br>  <br>
164  <br>  <br>
165  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
166  options for <b>pcre_exec()</b> have no Perl equivalents.  PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
167  <br>  <br>
168  <br>  <br>
169  (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF  (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
# Line 145  by the PCRE_BSR_ANYCRLF option. Line 177  by the PCRE_BSR_ANYCRLF option.
177  <br>  <br>
178  <br>  <br>
179  (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on  (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
180  different hosts that have the other endianness.  different hosts that have the other endianness. However, this does not apply to
181    optimized data created by the just-in-time compiler.
182  <br>  <br>
183  <br>  <br>
184  (k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a  (k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
# Line 170  Cambridge CB2 3QH, England. Line 203  Cambridge CB2 3QH, England.
203  REVISION  REVISION
204  </b><br>  </b><br>
205  <P>  <P>
206  Last updated: 11 September 2007  Last updated: 14 November 2011
207  <br>  <br>
208  Copyright &copy; 1997-2007 University of Cambridge.  Copyright &copy; 1997-2011 University of Cambridge.
209  <br>  <br>
210  <p>  <p>
211  Return to the <a href="index.html">PCRE index page</a>.  Return to the <a href="index.html">PCRE index page</a>.

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