Grails Validator for bank routing numbers

US Bank routing numbers have a checksum constraint built in. A Grails validator for that constraint is simple.

routingNum (blank:false, matches:/^\d{9}$/, 
            validator: {val, obj ->
  if (val?.size() < 9) return false
  def n = 0;
  for (def i = 0; i < val.size(); i += 3) {
    n += val[i].toInteger() * 3
    n += val[i + 1].toInteger() * 7
    n += val[i + 2].toInteger();
  }
  return (n != 0 && n % 10 == 0)
})