The Hostname Regex

Question

I\'m looking for the regex to validate hostnames. It must completely conform to the standard. Right now, I have

^[0-9a-z]([0-9a-z\\-]{0,61}[0-9a-z])?(\\

Answer 1

What about:

^(?=.{1,255})([0-9A-Za-z]|_{1}|\*{1}$)(?:(?:[0-9A-Za-z]|\b-){0,61}[0-9A-Za-z])?(?:\.[0-9A-Za-z](?:(?:[0-9A-Za-z]|\b-){0,61}[0-9A-Za-z])?)*\.?$

for matching only one '_' (for some SRV) at the beginning and only one * (in case of a label for a DNs wildcard)