This could be improved by defaulting args to [ :to_f ], thus eliminating the need for the ternary operator.

I couldn't really see a way to handle in_range in the same way as the others, without cracking open Integer as well, which I was trying to avoid. However, I've just realised my in_range method is broken.

I like the way you've extended Integer so that you can generalise the filter, but don't like the indiscriminate use of method_missing. Also, you could DRY up under and over.

This is the right idea, but has side effects, so can't be counted as "best practice".

Where does it say that the Matrix library is off-limits?

What if your hash looks like this?

{ :a => nil, 'a' => 'something' }

Brilliant! Wish I could mark this is "Best Practices" AND "Clever".

Doesn't this achieve the same thig?

(Matrix[[0,1],[1,1]] ** n)[1,0]

Nice trick to use a lambda, but using a recursive function to iterate over Fibonacci takes O(n^2) time.

What if you're given ']' as a comment marker?