Creating a meta proposal to bring together the discussion around #20, #19, #14, and #13. Feel free to correct me or close if I haven't thought about some edge cases in the individual locations that the pattern intends to be used, I'm less familiar with things like service worker scopes and CSP.

I'm making a couple of assumptions:

1. Using a single string is the simplest API for developers
2. Being incompatible with path-to-regexp is acceptable (on this point, it already is - just in a different way)
3. You don't use * (wildcards) as the default behavior
4. You enforce the scheme/hostname to be defined (e.g. http://, https?:// or *://)

With these assumptions, I think you could create a simpler API that mirrors the URL API. For example, new URLPattern('*', window.location) would be supported. You could always enforce a full URL instead of partial URLs, similar to the current URL API too.

In the service worker use-case, you can have it throw if the "origin" doesn't match the static prefix of the URLPattern. There was already some need for this sort of behavior due to the scope matching ordering: https://github.com/WICG/urlpattern/blob/master/explainer.md#scope-match-ordering.

Finally, on the path matching "magic", if you remove the magic prefix/suffix and make it explicit you can just do new URLPattern('/:foo{/bar}?', 'http://{:subdomain.}*.example.com').