Not sure if this is the place for this but here goes. Just a heads up, the solution to the lesson “Restrict Possible Usernames” requires a concept not taught yet. The solution requires quantity specifiers which aren’t taught until 2-3 lessons later. It may be worth considering moving the “Restrict Possible Usernames” lesson after the “Regular Expressions: Specify Only the Lower Number of Matches” lesson. Thanks to everyone who contributes to this amazing site!
The problem can be solved without quantity specifiers, but they sure make it easier. If you like, you can start a discussion as a GitHub Issue about whether this challenge should be moved.