make sure we are only doing sanity checking against regular expressions

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -114,7 +114,7 @@ module ActionDispatch
               requirements.reverse_merge!(@scope[:constraints]) if @scope[:constraints]
               @options.each { |k, v| requirements[k] = v if v.is_a?(Regexp) }
 
-              requirements.each do |_, requirement|
+              requirements.values.grep(Regexp).each do |requirement|
                 if requirement.source =~ %r{\A(\\A|\^)|(\\Z|\\z|\$)\Z}
                   raise ArgumentError, "Regexp anchor characters are not allowed in routing requirements: #{requirement.inspect}"
                 end

actionpack/test/controller/action_pack_assertions_test.rb

@@ -171,6 +171,14 @@ class ActionPackAssertionsControllerTest < ActionController::TestCase
     end
   end
 
+  def test_string_constraint
+    with_routing do |set|
+      set.draw do |map|
+        match "photos", :to => 'action_pack_assertions#nothing', :constraints => {:subdomain => "admin"}
+      end
+    end
+  end
+
   def test_assert_redirect_to_named_route_failure
     with_routing do |set|
       set.draw do