The regex used to validate username limits the extension dns name to 4 chars.
However, lot of domains exceed this rule.
The following python code give the longest dns suffix:

import urllib2

def is_ascii(s):
return all(ord(c) < 128 for c in s)

data = urllib2.urlopen("https://publicsuffix.org/list/public_suffix_list.dat")
nb_char = 0
longest_domain = None
for line in data:
if not line.startswith('//') and '.' not in line and is_ascii(line):
d = line.strip()
if len(d) > nb_char:
nb_char = len(d)
longest_domain = d

print("Longest domain {} with {} chars".format(longest_domain, nb_char))
The longest dns name is 18 chars