code: public static boolean isZipCode(String s) {
          if (isEmpty(s)) return defaultEmptyOK;
  
          String normalizedZip = stripCharsInBag(s, ZipCodeDelimiters);
  
          return (isInteger(normalizedZip) && ((normalizedZip.length() == digitsInZipCode1) || (normalizedZip.length() == digitsInZipCode2)));
      }
docs: isZIPCode returns true if string s is a valid U.S. ZIP code.  Must be 5 or 9 digits only.