Could someone please explain to me where/how this passes the last test where you have to only replace the last occurrence of the number? When I tried the replace method, foo99bar99 or whatever became foo100bar99 or foo100bar100. I can't figure out what part of this is addressing that.

I feel stupid, I don't understand the assignment. Could someone who has done this explain what it's asking us to do? I scrolled through many comments and found instructions about edge cases and complaints about vagueness but couldn't find an improved description for the whole assignment. Thank you!