In regards to the previous comment that says "there are several other differences including different meaning for the symbols ( [ different rules for which symbols need escaping", as far as I can tell from reading
http://www.tin.org/bin/man.cgi?section=7&topic=regex
there are absolutely no "other differences" except in what the man page calls "Obsolete ("basic'') regular expressions". However, PHP doesn't appear to use the obsolete form of expressions in its POSIX functions.
PCRE even supports all of the POSIX named character classes such as [:space:].