code: def validate_format(field, value, setting)
        result = value.nil? || setting.match(value).nil?
        add_message(field, "#{field.to_s} does not appear to be in the proper format.") if result
        return !result
      end
docs: Validates contents of field against a given Regexp. This can be tricky because you need
   to anchor both sides in most cases using \A and \Z to get a reliable match.