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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.
Most if not all posted solutions are incorrect.
A URL may have any number of subdomains, and may have a large variety of suffixes (see: https://publicsuffix.org/list/).
For example:
http://some.subdomain.mail.google.sth.ac.at
Is a valid URL. This project attempts to address the problem, but is clearly beyond the scope of this Kata (as it is marked #regex): https://github.com/lupomontero/psl.
The instructions should be modified to state what subdomains and suffixes are allowed.