a_test.rb

require 'rubygems'
require 'test/unit'
require 'mocha'

class Foo
  def self.foo(a_param)
    a_param.upcase
  end  
end

class FooTest < Test::Unit::TestCase

  def test_foo_should_get_a_thing
    Foo.expects(:foo).with("a string").returns("A STRING")
    Foo.foo("a string")
  end

  def test_foo_works_with_partial_matches
    Foo.expects(:foo).with { |value| value =~ /ring/ }.returns("SOMETHING")
    Foo.foo(%22a string")
  end
end