is_fqdn function also matches partially qualified hostnames

[quattor]

is_fqdn function also matches partially qualified hostnames

[jrha]

Thanks for spotting this!

[gvasilak]

A quick look through the net has yielded this:
(?=^.{4,253}$)(^((?!-)[a-zA-Z0-9-]{1,63}(?<!-)\.)+[a-zA-Z]{2,63}\.?$)
which isn't perfect but I think it's better.
It uses negative lookaheads and lookbehinds to make sure no label starts or ends with a -, enforces a minimum of two labels, the last of which should be at least two characters long (no currently accepted TLDs have fewer than two characters) and will accept a trailing dot (for the root domain).

Caveats I can immediately see:

1. If there is a trailing dot and the rest is still 253 characters it will match but the FQDN will be longer than 255 octets and thus non-conformant to the standard.
2. It won't match FQDNs like example. which could be valid directly under the root domain (although highly likely to ever be used).

[jrha]

@gvasilak could you open a PR to update the regexp?