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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.
It means that your regex doesn't match all and only multiple of threes.
There are some multiples of threes which would cause certain regex attempts to display false positives or false negatives.
I suggest you write a test to find out with which multiples of threes and non-multiples of threes your regex doesn't work correctly for, and then figure out how to fix it.