Regular Expression to Validate an email Address

if (ereg(^([a-zA-Z0-9_-.]+)@(([[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.)|(([a-zA-Z0-9-]+.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(]?)$, $emailstring)) { 
   echo "Email is valid";
} else {
   echo "Email is not valid";
}