diff --git a/actionpack/lib/action_dispatch/middleware/session/abstract_store.rb b/actionpack/lib/action_dispatch/middleware/session/abstract_store.rb
index 0c4ceb5c39..b03244d025 100644
--- a/actionpack/lib/action_dispatch/middleware/session/abstract_store.rb
+++ b/actionpack/lib/action_dispatch/middleware/session/abstract_store.rb
@@ -93,6 +93,13 @@ module ActionDispatch
         :cookie_only =>   true
       }
 
+      # This regular expression is used to split the levels of a domain:
+      # So www.example.co.uk gives:
+      # $1 => www.
+      # $2 => example
+      # $3 => co.uk
+      DOMAIN_REGEXP = /^(.*\.)*(.*)\.(...|...\...|....|..\...|..)$/
+
       def initialize(app, options = {})
         @app = app
         @default_options = DEFAULT_OPTIONS.merge(options)
@@ -123,7 +130,7 @@ module ActionDispatch
           end
 
           if options[:domain] == :all
-            env["HTTP_HOST"] =~ /^(.*\.)*(.*)\.(...|...\...|....|..\...)$/
+            env["HTTP_HOST"] =~ DOMAIN_REGEXP
             options[:domain] = ".#{$2}.#{$3}"
           end