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 "."
Afaics, the only URLs affected by the change (besides the one starting with + or -) are or the form where the domain consists of exactly two letters. Something like ``http://com`` would be treated as valid (I'll check that). But you're right. This is no longer related to this PR. I'll open a new issue to furtherdiscuss this.