foo_controller.rb

class FooController < ApplicationController
end

foo_controller_test.rb

require 'test_helper'

class FooControllerTest < ActionController::TestCase
  def test_route1
    assert_routing "/route1/abc/123",
      :controller  => 'foo',
      :action      => 'action1',
      :first_part  => 'abc',
      :second_part => '123'
  end

  def test_route2
    assert_routing "/route2/abc/123",
      :controller  => 'foo',
      :action      => 'action2',
      :first_part  => 'abc',
      :second_part => '123'
  end
end

routes.rb

Rails3app::Application.routes.draw do |map|
  match '/route1/:first_part/:second_part' => 'foo#action1', :constraints => { :first_part => /[a-z]+/  }
  match '/route2/:first_part/:second_part' => 'foo#action2', :constraints => { :first_part => /[a-z]+/i }
end

~output

$ rvm 1.9.1,1.8.7 rake test

ruby-1.9.1-p378: ruby 1.9.1p378 (2010-01-10 revision 26273) [i386-darwin10.2.0] 

(in /Users/aanand/src/rails3bugtest/rails3app)
Loaded suite /Users/aanand/.rvm/gems/ruby-1.9.1-p378%global/gems/rake-0.8.7/lib/rake/rake_test_loader
Started
..
Finished in 0.121878 seconds.

2 tests, 6 assertions, 0 failures, 0 errors, 0 skips

ruby-1.8.7-p249: ruby 1.8.7 (2010-01-10 patchlevel 249) [i686-darwin10.2.0] 

(in /Users/aanand/src/rails3bugtest/rails3app)
Loaded suite /Users/aanand/.rvm/gems/ruby-1.8.7-p249/gems/rake-0.8.7/lib/rake/rake_test_loader
Started
.F
Finished in 0.029549 seconds.

  1) Failure:
test_route2(FooControllerTest)
    []:
The recognized options <{"action"=>"action2", "first_part"=>"abc", "controller"=>"foo"}> did not match <{"second_part"=>"123",
 "action"=>"action2",
 "first_part"=>"abc",
 "controller"=>"foo"}>, difference: <{"second_part"=>"123"}>

2 tests, 4 assertions, 1 failures, 0 errors
Errors running test:functionals!