validating zip/Postal

var reg = /^\d*$/;
 var ch = null;
 try {
     if (reg.test(document.getElementById("arid1000000039").value)) {
         ch = "true";
     } else {
         ch = "false";
     }
 } catch (err) {
     ch = "Unrecognized character";
 }
 if (ch != null) {
     F(536870938).S(new CharType(ch));
 }