function checkZipCode(thisform) {
  // alert("Value : " + thisform.zipCode.value);
	
	var why = "";
    why += checkNotNull(thisform.zipCode.value);
    why += checkIsNumeric(thisform.zipCode.value);
    why += checkLength(thisform.zipCode.value);

	 

  if (why != "") {
       alert(why);
       return false;
    }
    
return true;
}

function checkNotNull(zipCode) {
 var error = "";
 if (zipCode == "") { error = "Zip Code wasn't entered.\n";}
    return error;
 }

 function checkIsNumeric(zipCode) 
 {
  var error = "";
  var numericExpression = /^[0-9]+$/;
	if(zipCode.match(numericExpression))
	{
		error = "";
	}
	else
	{
	error = "ZipCode must be numeric.\n";
	 
	}
	return  error;
 }
 
 function checkLength(zipCode) {
 var error = "";
 if (zipCode.length != 5){error = "Zip Code must be 5 characters in length.\n";}
   return error;
 }