Seconding the other reply this feels like (almost) an X/Y problem. At first skim over title and first sentence my off the cuff instinct was "Just try and resolve the name to an IP with Net::DNS, or gethostbyname and let your libc handle things" and be done with it. I could see several cases where some of your constraints while "RFC legal" wouldn't apply still (e.g. I've used a custom non-standard internal TLD for local names that's valid in the context I used it; (internal) DNS certainly would have resolved it but it would have failed bullet 2).

The cake is a lie.
The cake is a lie.
The cake is a lie.

In reply to Re: Regex for hostname validation by Fletch
in thread Regex for hostname validation by hrcerq

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