module SubdomainPrefixRouteExtension

  def self.included(base)
    base.alias_method_chain :recognition_conditions, :subdomain_prefix
  end

  def recognition_conditions_with_subdomain_prefix
    recognition_conditions_without_subdomain_prefix.tap do |result|
      result << subdomain_prefix_check if conditions[:subdomain_prefix]
    end
  end

  private

    def subdomain_prefix_check
      %{env[:subdomain] =~ /^#{ conditions[:subdomain_prefix] }(?:\.|$)/}
    end

end

ActionController::Routing::Route.send :include, SubdomainPrefixRouteExtension


# config/routes.rb
map.root :controller => 'items', :action => 'index', :subdomain_prefix => 'my'