This project is a US phone number validator app that tells you whether a phone number is valid or not. It takes the phone number as input, parses it, and then outputs the result. Pretty straightforward on the surface, not so much underneath.
This one was very difficult for me. It involved a lot of complex regex pattern detection--which was really good--but I would be lying if I said I remember how to do it now. Not to worry though, because 'what the hand does, the mind remembers'.
Even though I'm from Australia so this tool isn't really applicable to me, I found it incredibly helpful, interesting and insightful. It was a great learning experience and I'm glad I did it.