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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 agree, this is not a good question right now. Without a given list of second level domains it is not possible to extract the "domain" (which is also not the correct name as pointed out by others). For example: www.museum.tt should return "www", because "mesuem.tt" is actually a SLD that can be registered (same as .co.uk). There is a npm packages that solves this problem:
https://github.com/lupomontero/pslAs mentioned it contains a complete list of all registerable SLD's.