Add constraint to route to exclude certain keyword

Question

I am using Rails and I want to use contraint in route to exclude that route if keyword "incident" is anywhere in the url.

I am using rails3.

Here is my existing routes.

match ':arg', :to => "devices#show", :constraints => {:arg => /???/} 

I need to put something in constraints so that it does not match if word "incident" is there.

Thanks