def evaluate(actual, expected)
      expected = %r[#{Regexp.escape(expected)}] if expected.kind_of?(String)
      if actual.to_s =~ expected
        pass(new_message.matches(expected))
      else
        fail(expected_message(expected).to_match(actual))
      end
    end
(see Riot::AssertionMacro#evaluate)
@param [Regex, String] expected the string or regex to be used in comparison