function isValidZip(num) { return (/^\d{5}$/.test(num)); }
  function showMessage() { alert("Please enter a valid zip code."); }

  function getForm() {
    var zip       = $F("address1_zip");
    var senior    = ($("is_over65_1").checked) ? $F("is_over65_1") : $F("is_over65_0");

    var params  = "toolbar=yes,menubar=yes,location=yes,scrollbars=yes,resizable=yes,status=yes,";
    params     += "width=" + screen.width;
    params     += ",height=" + screen.height;
    params     += ",fullscreen=yes"
    var pops    = "popup_handler.php" + "?zip=" + zip + "&over65=" + senior;
    window.open(pops, "", params);
  }
  
  Event.observe(window, 'load', function() {
    Event.observe('submit', 'click', function(e){
      if(!isValidZip($F("address1_zip"))){ showMessage(); Event.stop(e); }
    });
  });