what is d braceopening 4 braceclosing in python?

I am learning python and views in Django. What represents this d in the following code url(r'^articles/(?Pd{4})/(?Pd{2})/$','list_articles'), # views for year and months

I think it 's d4 for 2016 for example and d2 for 06 , but what is the d with those braces. Thanks.

ps : sorry for the title, it was not accepting the { and } characters