protected String validate(String zipCode) {
        Pattern REGEX = Pattern.compile("^\\d{5}(-\\d{4})?$");

        if (zipCode == null) {
            throw new NullPointerException("Zip Code cannot be null!");
        }

        if (!strictMode || REGEX.matcher(zipCode).matches()) {
            return zipCode;
        }

        throw new IllegalArgumentException("Invalid zip code format specified: " + zipCode);
    }