Following up to Regular expression to match hostname or IP Address? and using Restrictions on valid host names as a reference, what is the most readable, concise way to match/validate a hostname/fqdn (fully qualified domain name) in Python? I've answered with my attempt below, improvements welcome.
import re
def is_valid_hostname(hostname):
if len(hostname) > 255:
return False
if hostname[-1] == ".":
hostname = hostname[:-1] # strip exactly one dot from the right, if present
allowed = re.compile("(?!-)[A-Z\d-]{1,63}(?<!-)$", re.IGNORECASE)
return all(allowed.match(x) for x in hostname.split("."))
ensures that each segment
It also avoids double negatives (not disallowed
), and if hostname
ends in a .
, that's OK, too. It will (and should) fail if hostname
ends in more than one dot.