Annotation of embedaddon/pcre/doc/html/pcrecompat.html, revision 1.1.1.3
1.1 misho 1: <html>
2: <head>
3: <title>pcrecompat specification</title>
4: </head>
5: <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6: <h1>pcrecompat man page</h1>
7: <p>
8: Return to the <a href="index.html">PCRE index page</a>.
9: </p>
10: <p>
11: 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: <br>
15: <br><b>
16: DIFFERENCES BETWEEN PCRE AND PERL
17: </b><br>
18: <P>
19: 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
21: versions 5.10 and above.
22: </P>
23: <P>
1.1.1.2 misho 24: 1. PCRE has only a subset of Perl's Unicode support. Details of what it does
25: have are given in the
1.1 misho 26: <a href="pcreunicode.html"><b>pcreunicode</b></a>
27: page.
28: </P>
29: <P>
30: 2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
31: not mean what you might think. For example, (?!a){3} does not assert that the
32: next three characters are not "a". It just asserts that the next character is
33: 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>
37: <P>
38: 3. Capturing subpatterns that occur inside negative lookahead assertions are
39: 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: 4. Though binary zero characters are supported in the subject string, they are
46: not allowed in a pattern string because it is passed as a normal C string,
47: terminated by zero. The escape sequence \0 can be used in the pattern to
48: represent a binary zero.
49: </P>
50: <P>
51: 5. The following Perl escape sequences are not supported: \l, \u, \L,
52: \U, and \N when followed by a character name or Unicode value. (\N on its
53: own, matching a non-newline character, is supported.) In fact these are
54: 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>
59: <P>
60: 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
62: 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
64: 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>
69: <P>
70: 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 $
77: 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
79: following examples:
80: <pre>
81: Pattern PCRE matches Perl matches
82:
83: \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
84: \Qabc\$xyz\E abc\$xyz abc\$xyz
85: \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
86: </pre>
87: The \Q...\E sequence is recognized both inside and outside character classes.
88: </P>
89: <P>
90: 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
91: constructions. However, there is support for recursive patterns. This is not
92: 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
94: the
95: <a href="pcrecallout.html"><b>pcrecallout</b></a>
96: documentation for details.
97: </P>
98: <P>
99: 10. Subpatterns that are called as subroutines (whether or not recursively) are
100: 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>
109: <P>
1.1.1.3 ! misho 110: 11. If any of the backtracking control verbs are used in an assertion or in a
! 111: subpattern that is called as a subroutine (whether or not recursively), their
! 112: effect is confined to that subpattern; it does not extend to the surrounding
! 113: pattern. This is not always the case in Perl. In particular, if (*THEN) is
! 114: present in a group that is called as a subroutine, its action is limited to
! 115: that group, even if the group does not contain any | characters. There is one
! 116: exception to this: the name from a *(MARK), (*PRUNE), or (*THEN) that is
! 117: encountered in a successful positive assertion <i>is</i> passed back when a
! 118: match succeeds (compare capturing parentheses in assertions). Note that such
! 119: subpatterns are processed as anchored at the point where they are tested.
1.1 misho 120: </P>
121: <P>
122: 12. There are some differences that are concerned with the settings of captured
123: strings when part of a pattern is repeated. For example, matching "aba" against
124: the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
125: </P>
126: <P>
127: 13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
128: names is not as general as Perl's. This is a consequence of the fact the PCRE
129: works internally just with numbers, using an external table to translate
130: between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
131: where the two capturing parentheses have the same number but different names,
132: is not supported, and causes an error at compile time. If it were allowed, it
133: would not be possible to distinguish which parentheses matched, because both
134: names map to capturing subpattern number 1. To avoid this confusing situation,
135: an error is given at compile time.
136: </P>
137: <P>
138: 14. Perl recognizes comments in some places that PCRE does not, for example,
139: between the ( and ? at the start of a subpattern. If the /x modifier is set,
1.1.1.3 ! misho 140: Perl allows white space between ( and ? but PCRE never does, even if the
1.1 misho 141: PCRE_EXTENDED option is set.
142: </P>
143: <P>
144: 15. PCRE provides some extensions to the Perl regular expression facilities.
145: Perl 5.10 includes new features that are not in earlier versions of Perl, some
146: of which (such as named parentheses) have been in PCRE for some time. This list
147: is with respect to Perl 5.10:
148: <br>
149: <br>
150: (a) Although lookbehind assertions in PCRE must match fixed length strings,
151: each alternative branch of a lookbehind assertion can match a different length
152: of string. Perl requires them all to have the same length.
153: <br>
154: <br>
155: (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
156: meta-character matches only at the very end of the string.
157: <br>
158: <br>
159: (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
160: meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
161: (Perl can be made to issue a warning.)
162: <br>
163: <br>
164: (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
165: inverted, that is, by default they are not greedy, but if followed by a
166: question mark they are.
167: <br>
168: <br>
169: (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
170: only at the first matching position in the subject string.
171: <br>
172: <br>
173: (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
174: PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
175: <br>
176: <br>
177: (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
178: by the PCRE_BSR_ANYCRLF option.
179: <br>
180: <br>
181: (h) The callout facility is PCRE-specific.
182: <br>
183: <br>
184: (i) The partial matching facility is PCRE-specific.
185: <br>
186: <br>
187: (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
188: different hosts that have the other endianness. However, this does not apply to
189: optimized data created by the just-in-time compiler.
190: <br>
191: <br>
1.1.1.2 misho 192: (k) The alternative matching functions (<b>pcre_dfa_exec()</b> and
193: <b>pcre16_dfa_exec()</b>) match in a different way and are not Perl-compatible.
1.1 misho 194: <br>
195: <br>
196: (l) PCRE recognizes some special sequences such as (*CR) at the start of
197: a pattern that set overall options that cannot be changed within the pattern.
198: </P>
199: <br><b>
200: AUTHOR
201: </b><br>
202: <P>
203: Philip Hazel
204: <br>
205: University Computing Service
206: <br>
207: Cambridge CB2 3QH, England.
208: <br>
209: </P>
210: <br><b>
211: REVISION
212: </b><br>
213: <P>
1.1.1.3 ! misho 214: Last updated: 01 June 2012
1.1 misho 215: <br>
1.1.1.2 misho 216: Copyright © 1997-2012 University of Cambridge.
1.1 misho 217: <br>
218: <p>
219: Return to the <a href="index.html">PCRE index page</a>.
220: </p>
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