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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
I love your solution because of the comments and the way you build the regex. I have a hard time understanding how regex workes when it and this clearly explains a lot (like why do you use "?" signs after the starting and ending letter and why do you use "*" at the letters between). ^^
Pretty nice kata, but if you make a state about invalid charcters you should make a test for it, encurageing those who make this kata to handle such outcome.