regular countenance uses an "or" conditional

i assistance minute unchanging expression. django application, users strike following url:

http://www.example.com/a1/b2/c3

i'd emanate unchanging countenance allows accepts any following current url:

http://www.example.com/a1  
http://www.example.com/a1/b2  
http://www.example.com/a1/b2/c3  

i'm guessing i need "or" conditional, nonetheless i'm carrying problem removing regex validate. any thoughts?

update: here regex far. note i have enclosed "" apportionment -- django handles me. i'm only concerned validating 1,2, 3 subdirectories.

^(\w{1,20})|((\w{1,20})/(\w{1,20}))|((\w{1,20})/(\w{1,20})/(\w{1,20}))$