Validate a hostname string

Question

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 matc

Answer 1

Complimentary to the @TimPietzcker answer. Underscore is a valid hostname character (but not for domain name) . While double dash is commonly found for IDN punycode domain(e.g. xn--). Port number should be stripped. This is the cleanup of the code.

import re
def is_valid_hostname(hostname):
    if len(hostname) > 255:
        return False
    hostname = hostname.rstrip(".")
    allowed = re.compile("(?!-)[A-Z\d\-\_]{1,63}(?