| author | Tom Sutcliffe <thomas.sutcliffe@accenture.com> |
| Sat, 31 Jul 2010 19:07:57 +0100 | |
| changeset 23 | 092bcc217d9d |
| parent 0 | 7f656887cf89 |
| permissions | -rw-r--r-- |
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<html> |
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<head> |
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<title>pcrecompat specification</title> |
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</head> |
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"> |
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<h1>pcrecompat man page</h1> |
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<p> |
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Return to the <a href="index.html">PCRE index page</a>. |
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</p> |
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parents:
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<p> |
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This page is part of the PCRE HTML documentation. It was generated automatically |
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from the original man page. If there is any nonsense in it, please consult the |
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man page, in case the conversion went wrong. |
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<br> |
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<br><b> |
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DIFFERENCES BETWEEN PCRE AND PERL |
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</b><br> |
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<P> |
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This document describes the differences in the ways that PCRE and Perl handle |
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regular expressions. The differences described here are mainly with respect to |
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Perl 5.8, though PCRE versions 7.0 and later contain some features that are |
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expected to be in the forthcoming Perl 5.10. |
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</P> |
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parents:
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<P> |
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1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what |
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it does have are given in the |
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<a href="pcre.html#utf8support">section on UTF-8 support</a> |
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in the main |
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<a href="pcre.html"><b>pcre</b></a> |
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page. |
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</P> |
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<P> |
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2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits |
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them, but they do not mean what you might think. For example, (?!a){3} does
|
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not assert that the next three characters are not "a". It just asserts that the |
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next character is not "a" three times. |
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</P> |
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parents:
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<P> |
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3. Capturing subpatterns that occur inside negative lookahead assertions are |
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counted, but their entries in the offsets vector are never set. Perl sets its |
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numerical variables from any such patterns that are matched before the |
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assertion fails to match something (thereby succeeding), but only if the |
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negative lookahead assertion contains just one branch. |
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</P> |
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<P> |
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4. Though binary zero characters are supported in the subject string, they are |
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not allowed in a pattern string because it is passed as a normal C string, |
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terminated by zero. The escape sequence \0 can be used in the pattern to |
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represent a binary zero. |
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</P> |
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<P> |
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5. The following Perl escape sequences are not supported: \l, \u, \L, |
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\U, and \N. In fact these are implemented by Perl's general string-handling |
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and are not part of its pattern matching engine. If any of these are |
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encountered by PCRE, an error is generated. |
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</P> |
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parents:
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<P> |
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6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is |
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built with Unicode character property support. The properties that can be |
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First submission to Symbian Foundation staging server.
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tested with \p and \P are limited to the general category properties such as |
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Lu and Nd, script names such as Greek or Han, and the derived properties Any |
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and L&. |
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</P> |
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<P> |
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7. PCRE does support the \Q...\E escape for quoting substrings. Characters in |
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between are treated as literals. This is slightly different from Perl in that $ |
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and @ are also handled as literals inside the quotes. In Perl, they cause |
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variable interpolation (but of course PCRE does not have variables). Note the |
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following examples: |
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<pre> |
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Pattern PCRE matches Perl matches |
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72 |
|
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\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz |
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\Qabc\$xyz\E abc\$xyz abc\$xyz |
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\Qabc\E\$\Qxyz\E abc$xyz abc$xyz |
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</pre> |
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The \Q...\E sequence is recognized both inside and outside character classes. |
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</P> |
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<P> |
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8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
|
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constructions. However, there is support for recursive patterns. This is not |
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available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout" |
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feature allows an external function to be called during pattern matching. See |
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the |
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<a href="pcrecallout.html"><b>pcrecallout</b></a> |
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documentation for details. |
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</P> |
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88 |
<P> |
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parents:
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9. Subpatterns that are called recursively or as "subroutines" are always |
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treated as atomic groups in PCRE. This is like Python, but unlike Perl. |
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91 |
</P> |
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92 |
<P> |
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93 |
10. There are some differences that are concerned with the settings of captured |
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94 |
strings when part of a pattern is repeated. For example, matching "aba" against |
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the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b". |
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96 |
</P> |
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97 |
<P> |
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98 |
11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F), |
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(*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an |
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argument. PCRE does not support (*MARK). If (*ACCEPT) is within capturing |
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101 |
parentheses, PCRE does not set that capture group; this is different to Perl. |
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102 |
</P> |
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103 |
<P> |
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104 |
12. PCRE provides some extensions to the Perl regular expression facilities. |
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105 |
Perl 5.10 will include new features that are not in earlier versions, some of |
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106 |
which (such as named parentheses) have been in PCRE for some time. This list is |
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107 |
with respect to Perl 5.10: |
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108 |
<br> |
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109 |
<br> |
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110 |
(a) Although lookbehind assertions must match fixed length strings, each |
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111 |
alternative branch of a lookbehind assertion can match a different length of |
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112 |
string. Perl requires them all to have the same length. |
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113 |
<br> |
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114 |
<br> |
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115 |
(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ |
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meta-character matches only at the very end of the string. |
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117 |
<br> |
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118 |
<br> |
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119 |
(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special |
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meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored. |
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121 |
(Perl can be made to issue a warning.) |
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122 |
<br> |
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123 |
<br> |
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124 |
(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is |
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125 |
inverted, that is, by default they are not greedy, but if followed by a |
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126 |
question mark they are. |
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127 |
<br> |
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128 |
<br> |
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129 |
(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried |
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130 |
only at the first matching position in the subject string. |
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131 |
<br> |
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132 |
<br> |
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133 |
(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE |
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134 |
options for <b>pcre_exec()</b> have no Perl equivalents. |
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135 |
<br> |
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136 |
<br> |
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137 |
(g) The \R escape sequence can be restricted to match only CR, LF, or CRLF |
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138 |
by the PCRE_BSR_ANYCRLF option. |
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139 |
<br> |
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140 |
<br> |
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141 |
(h) The callout facility is PCRE-specific. |
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142 |
<br> |
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143 |
<br> |
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144 |
(i) The partial matching facility is PCRE-specific. |
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145 |
<br> |
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146 |
<br> |
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147 |
(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on |
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148 |
different hosts that have the other endianness. |
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149 |
<br> |
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150 |
<br> |
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151 |
(k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a |
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152 |
different way and is not Perl-compatible. |
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153 |
<br> |
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154 |
<br> |
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155 |
(l) PCRE recognizes some special sequences such as (*CR) at the start of |
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156 |
a pattern that set overall options that cannot be changed within the pattern. |
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157 |
</P> |
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158 |
<br><b> |
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159 |
AUTHOR |
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160 |
</b><br> |
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161 |
<P> |
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162 |
Philip Hazel |
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163 |
<br> |
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164 |
University Computing Service |
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165 |
<br> |
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166 |
Cambridge CB2 3QH, England. |
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167 |
<br> |
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168 |
</P> |
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169 |
<br><b> |
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170 |
REVISION |
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171 |
</b><br> |
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172 |
<P> |
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173 |
Last updated: 11 September 2007 |
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174 |
<br> |
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175 |
Copyright © 1997-2007 University of Cambridge. |
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176 |
<br> |
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177 |
<p> |
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178 |
Return to the <a href="index.html">PCRE index page</a>. |
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179 |
</p> |