function isNumeric(numberString)  {
   var validNumbers = "0123456789";
   var result = true;

   if (numberString.length == 0 || isNaN(numberString) ) return false;

   //  test strString consists of valid characters listed above
   for (i = 0; i < numberString.length && result == true; i++)  {
      var strChar = numberString.charAt(i);
      if (validNumbers.indexOf(strChar) == -1)  {
         result = false;
         }
      }
   return result;
 }