def call(target)
        value = target.validation_property_value(field_name)
        return true if @options[:allow_nil] && value.nil?
        validation = @options[:as] || @options[:with]
        raise "No such predefined format '#{validation}'" if validation.is_a?(Symbol) && !FORMATS.has_key?(validation)
        validator = validation.is_a?(Symbol) ? FORMATS[validation][0] : validation
        valid = case validator
          when Proc   then validator.call(value)
          when Regexp then value =~ validator
          else
            raise UnknownValidationFormat, "Can't determine how to validate #{target.class}##{field_name} with #{validator.inspect}"
        end
        return true if valid
        error_message = @options[:message] || ValidationErrors.default_error_message(:invalid, field_name)
        field = CouchRest.humanize(field_name)
        error_message = error_message.call(field, value) if error_message.respond_to?(:call)
        add_error(target, error_message, field_name)
        false
      end