What does .:format mean in rake routes

Question

I type rake routes and I get a bunch of urls like this - /articles/:id(.:format)

My question is - what does the .:format mean? It is not clear

Answer 1

.:format matches a mime type.

For instance if you send a request looking for index.html the format catches 'html' as :format.

Then in your controller it will get processed by something like

respond_to do |format|
  format.html { #do something like redirect in here }
end