Ruby on Rails gem to make passwords more secure?

Ruby on Rails gem to make passwords more secure?

I built a Ruby on Rails authentication system from scratch and it works
great.
(I am aware of the existence of Devise, yet I wanted to write my own
system to get a better grasp of Ruby on Rails in general.)
The only thing that worries me is users who pick words from dictionaries,
sequential numbers (e.g. "12345679"), their own user names, or the
all-time classic "password" as their own password.
Is there a gem that validates against any of those? Or will I have to work
with regular expressions here?
Thanks for any help.