<?php
if (! preg_match('/^\d{5}(-\d{4})?$/',
                  $_POST['zipcode'])) { ... }

$email_rx = '/^[^@\s]+@([-a-z0-9]+\.)+[a-z]{2,}$/i';
if (! preg_match($email_rx, $_POST['email'])) { ... }
?>