Make a route work for all subdomains and also when no subdomain provided?

Make a route work for all subdomains and also when no subdomain provided?

I got an application that uses subdomain but also can be accessed without
a subdomain. I got a few paths that need to be accessible all the time,
whether a subdomain is provided or not.
This is what I did to make it work for all subdomains:
constraints :subdomain => /.+/ do
...
end
How can I make this also accessible when no subdomain at all is provided?