Rails routes with prefix before the :id

This works: map.connect '/bananas/pdf/file/:id.pdf', :controller => 'bananas', :action => 'pdf'

This doesn't work: map.connect '/bananas/pdf/file_:id.pdf', :controller => 'bananas', :action => 'pdf'

So once I change the slash to underscore the route doesn't work the way I expect it to. Is there some way I need to escape the routes to make them work (Rails 2.1.2)?