class ActiveModel::Validations::FormatValidator

def check_validity!
  unless options.include?(:with) ^ options.include?(:without)  # ^ == xor, or "exclusive or"
    raise ArgumentError, "Either :with or :without must be supplied (but not both)"
  end
  if options[:with] && !options[:with].is_a?(Regexp)
    raise ArgumentError, "A regular expression must be supplied as the :with option of the configuration hash"
  end
  if options[:without] && !options[:without].is_a?(Regexp)
    raise ArgumentError, "A regular expression must be supplied as the :without option of the configuration hash"
  end
end