diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index fc1fbfd40ecc8..96cd1aa2ccd95 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -355,7 +355,7 @@ def split_to(to)
 
           def add_controller_module(controller, modyoule)
             if modyoule && !controller.is_a?(Regexp)
-              if controller && controller.start_with?("/")
+              if %r{\A/}.match?(controller)
                 controller[1..-1]
               else
                 [modyoule, controller].compact.join("/")