What are the [] in the regular expression for?
ereg( "Mozilla/4.0 (compatible; MSIE 5.5; Windows 98; Win 9x 4.90)", $HTTP_USER_AGENT ))
is what you want. What you have is exactly the same as
ereg( "[ ()./;04589abcdeEiIlMmnopStwxz]", $HTTP_USER_AGENT ))
which matches any string that starts with any one of the characters in the []