there are several other differences including different meaning
for the symbols ( [ different rules for which symbols need
escaping (they can't be the same as both standard posix and
extended posix) you should read the full documentation for PCRE
before chaging any posix regex to use pcre. Differences from
POSIX regex As of PHP 5.3.0, the POSIX Regex extension is
deprecated. There are a number of differences between POSIX
regex and PCRE regex. This page lists the most notable ones that
are necessary to know when converting to PCRE. - The PCRE
functions require that the pattern is enclosed by delimiters. -
Unlike POSIX, the PCRE extension does not have dedicated
functions for case-insensitive matching. Instead, this is
supported using the i (PCRE_CASELESS) pattern modifier. Other
pattern modifiers are also available for changing the matching
strategy. - The POSIX functions find the longest of the leftmost
match, but PCRE stops on the first valid match. If the string
doesn't match at all it makes no difference,