<?php
$pattern = '/^([a-z0-9])(([-a-z0-9._])*([a-z0-9]))*\@([a-z0-9])' .
           '(([a-z0-9-])*([a-z0-9]))+' .
           '(\.([a-z0-9])([-a-z0-9_-])?([a-z0-9])+)+$/i';

echo preg_match ($pattern, "email-address-to-validate@host.tld");
?>