How to check if string follows a specific format?

All,

I am wondering how to write an "if" statement that check if a string follows a specific format.

Imagine the following:

if username == 'user1' or \
   username == 'user2' or \
   username == 'user3' or \
   username == 'user4' or \
   username == 'user5' or \
   username == 'user6' or \
   username == 'user7' or \
   username == 'user8' or \
   username == 'user9' or \
   username == 'user10' or \
   username == 'user11' or \
   username == 'user12' or \
   username == 'user13' or \
   username == 'user14' or \
   username == 'user15' or \
   username == 'user16' or \
   username == 'user17' or \
   username == 'user18' or \
   username == 'user19' or \
   username == 'user20' or \
   username == 'user21' or \
   username == 'user22' or \
   username == 'user23' or \
   username == 'user24' or \
   username == 'user25' or \
   username == 'user26' or \

and so on.

Obviously, I cannot write out a condition for every number out there...

How can I write some parameters that checks if the input for "username" contains 'user', then followed with an integer converted into a string?

Thanks, and sorry if the question is vague, I am new to Python.