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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.
Can you explain the c => '5987604321'[c] , how does that work?
Good to know thank you. Ill try remember for future, I do need to improve on my Regex knowledge.
You can just right a lower case "i" right after the last slash of your regex expression to make it check for the letters you chose of any case (lower or upper), so that you don't have to write the uppercase version too.