No need for a regexp here.

actionpack/lib/action_view/helpers/asset_paths.rb

@@ -21,7 +21,6 @@ module ActionView
         return source if is_uri?(source)
 
         source = rewrite_extension(source, dir, ext) if ext
-        source = "/#{dir}/#{source}" unless source[0] == ?/
         source = rewrite_asset_path(source, dir)
 
         if controller && include_host

actionpack/lib/action_view/helpers/asset_tag_helpers/asset_paths.rb

@@ -41,7 +41,8 @@ module ActionView
 
         # Break out the asset path rewrite in case plugins wish to put the asset id
         # someplace other than the query string.
-        def rewrite_asset_path(source, path = nil)
+        def rewrite_asset_path(source, dir)
+          source = "/#{dir}/#{source}" unless source[0] == ?/
           path = config.asset_path
 
           if path && path.respond_to?(:call)

actionpack/lib/action_view/helpers/sprockets_helper.rb

@@ -40,10 +40,10 @@ module ActionView
 
       class AssetPaths < ActionView::Helpers::AssetPaths #:nodoc:
         def rewrite_asset_path(source, dir)
-          if source =~ /^\/#{dir}\/(.+)/
-            assets.path($1, performing_caching?, dir)
-          else
+          if source[0] == ?/
             source
+          else
+            assets.path(source, performing_caching?, dir)
           end
         end