I saw some floats provided, so I chose to reject strings rather than select integers.

You could have used Kernerl.max/2 ( https://hexdocs.pm/elixir/Kernel.html#max/2 ) for a performance boost :D

A little nitpicky, but you only need to run &block on each element once, not twice: find list.select(&block) as var, and then return as the second element all elements in list and not in var. If &block is expensive, it might be desirable not to run it twice.

Clean and readable solution

takes a list of non-negative integers and strings

I'd assume that when we remove all the strings, the only things left are the integers.

Technically wrong, as the specs said: "remove all the strings". The tests are not good enough to catch that though.

How does \1 work? I've tried \0 and received [], tried \2 and received invalid backref number/name: /(.)\2/. Why \1 return duplicates?