my $hname_re = qr/
	^ (?=(?&validchar){1,255}$) (?!\d+$)
		(?&label)
		(?: (?:\.(?&label))* \.(?&tld) \.? )?
	$
	(?(DEFINE)
		(?<validchar>[a-zA-Z0-9.-])
		(?<alnum>[a-zA-Z0-9])
		(?<alnumdash>[a-z-A-Z0-9])

		(?<label>(?> (?&alnum)
				(?: (?&alnumdash){,61}
				    (?&alnum) )? ) )

		(?<tld>(?!(\d+|.)\.?$) (?&label) )
	)
/x;