zabbix frontend uses perl regexps, server - posix extended.
on the frontend, posix-e is deprecated since 5.3.0 (according to http://www.php.net/manual/en/intro.regex.php)
this is bad because of inconsistency, but especially bad for cases when both frontend and server are supposed to do the same thing, for example, global regexps (testing in frontend and actual use in server)
keywords : pcre
note that this concerns all daemons, not just server - proxy, agent and java gateway should be verified, too