// JavaScript Document
function checkform() {
 var
  zipcode = document.forms[0].zip.value.toUpperCase().split(' ').join(''),
  rExp;

 document.forms[0].zipcode.value = zipcode + '|30';

rExp = /^[1-9]\d{3}$/;
 if (!rExp.exec(zipcode)) {
  alert('Voer alleen de 4 cijfers van uw postcode in');
  return false;
  };  
}