// validate zip code (zip code is not required)

function checkZipcode (strng) {
var error="";
   if (!(strng)) {
        error = "Please enter a valid zip code.\n";
   }

   for (i = 0; i<strng.length; i++) {
      if (strng.charAt(i) < "0" || strng.charAt(i) > "9") {
          error = "The zip code contains illegal characters.\n";
      }
   }
 return error;
 }