validate email address when submitting a form

function validateEmail ( fieldId ) {
  var field = document.getElementById( fieldId );
  var address = field.value;
  //based on 'valid email' by krisdb via snipplr.com
  var filter=/^.+@.+\..{2,3}$/;
  return (function (){
    if (filter.test(address))
      return true;
    else {
      if (address == '') { 
	alert('Please enter a valid email address before submitting the form.');
      } else {
	alert('"' + address + '" is not a valid email address.  Please correct your address and try again.');
      }
      field.focus();
      return false;
    }
  })();
}
window.onload = function() { 
  clearOnInitialFocus('email');
  document.forms[0].onsubmit = function () { return validateEmail('email') };
}