diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 0c77183dc12e6faf65809e3d0d9a42c999a024f6..76c7ec35b9f917c8c3a578037cd5993a795b4160 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -726,7 +726,7 @@ def match(*args)
           path = options.delete(:path)
           action = args.first
 
-          if action && action.to_s =~ /^[\w_]+$/
+          if action && action.to_s =~ /^[A-Za-z_]\w*$/
             path = path_for_action(action, path)
             options[:action] ||= action
             options[:as] = name_for_action(action, options[:as])
diff --git a/actionpack/test/dispatch/routing_test.rb b/actionpack/test/dispatch/routing_test.rb
index a84c4d2d1806c4509f30aa5a693d536c87fb940d..3f51798285d32d01cf3828fcee2df602762c5c5e 100644
--- a/actionpack/test/dispatch/routing_test.rb
+++ b/actionpack/test/dispatch/routing_test.rb
@@ -188,7 +188,9 @@ def self.matches?(request)
         end
       end
 
-      resources :sheep
+      resources :sheep do
+        get "_it", :on => :member
+      end
 
       resources :clients do
         namespace :google do
@@ -996,6 +998,7 @@ def test_resources_for_uncountable_names
       assert_equal '/sheep/1', sheep_path(1)
       assert_equal '/sheep/new', new_sheep_path
       assert_equal '/sheep/1/edit', edit_sheep_path(1)
+      assert_equal '/sheep/1/_it', _it_sheep_path(1)
     end
   end