For work I had to write a custom url model field. This model field when setting up accepts a default protocol, and a list of other protocols.

When checking the protocol, the url is split by "://". If the split has one or two parts, then the url is validly formed.

In the event of a single element split, there is no protocol specified. When there is no protocol, the url is prepended with the default protocol specified. If there is a protocol, it is checked to make sure it exists in a union of the default protocol and other protocols …