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Regular Expression: Posix Extended Regular Expression
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These functions all take a regular expression string as their first argument. PHP uses the POSIX extended regular expressions as defined by POSIX 1003.2. For a full description of POSIX regular expressions see the regex man pages included in the regex directory in the PHP distribution. It's in manpage format, so you'll want to do something along the lines of man /usr/local/src/regex/regex.7 in order to read it.
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The meaning of metacharacters escaped with a backslash is reversed for some characters in the POSIX Extended Regular Expression (ERE) syntax. With this syntax, a backslash causes the metacharacter to be treated as a literal character. Additionally, support is removed for \n backreferences and the following metacharacters are added:
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Google Code Search supports POSIX extended regular expression syntax, excluding backreferences, collating elements, and collation classes. To search for a space character, escape it with a backslash, as in hello,\ world. You can search for literal strings by enclosing the strings in quotation marks, as in "hello, world".
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Posix Extended Regular Expression