It's actually more than this - see 90d2802b71a6e89aedfe40564a37bd35f777e541 for what can be used. Basically it's String, Regexp, Array or anything that responds to `===`. So for example you could specify a port range: e.g:

``` ruby
get '/', to: 'pages#index', constraints: { :port =>  8000..8002  }
```