Gists from my blog post http://www.codelord.net/2012/02/04/extend-your-toolbox-custom-matchers/

beautiful_and_dry.rb

commentor.should_receive(:add).with(anonymous_comment)

intent.rb

# This
response['X-Runtime'].should =~ /[\d\.]+/

# .. or this?
response['X-Runtime'].should be_a_number

rspec_matcher.rb

RSpec::Matchers.define :be_a_multiple_of do |expected|
  match do |actual|
    actual % expected == 0
  end
end

sucky_non_dry.rb

expected_comment = Comment.new(anonymous: true, user: "the dude", reply_to: nil)
commentor.should_receive(:add).with(expected_comment)