Validate urls like http://.com or http://..com

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11702
| License       | MIT

This PR fixes the regular expression to accept valid URLs. It does not allow to start a domain name with "-" or "."
But a domain like ``a.com`` has been valid before and wouldn't be valid now if I don't miss anything.