Validate a hostname string

Don't reinvent the wheel. You can use a library, e.g. validators. Or you can copy their code:

Installation

pip install validators

Usage

import validators
if validators.domain('example.com')
    print('this domain is valid')

Here's a bit stricter version of Tim Pietzcker's answer with the following improvements:

• Limit the length of the hostname to 253 characters (after stripping the optional trailing dot).
• Limit the character set to ASCII (i.e. use [0-9] instead of \d).
• Check that the TLD is not all-numeric.

import re

def is_valid_hostname(hostname):
    if hostname[-1] == ".":
        # strip exactly one dot from the right, if present
        hostname = hostname[:-1]
    if len(hostname) > 253:
        return False

    labels = hostname.split(".")

    # the TLD must be not all-numeric
    if re.match(r"[0-9]+$", labels[-1]):
        return False

    allowed = re.compile(r"(?!-)[a-z0-9-]{1,63}(?<!-)$", re.IGNORECASE)
    return all(allowed.match(label) for label in labels)

Per The Old New Thing, the maximum length of a DNS name is 253 characters. (One is allowed up to 255 octets, but 2 of those are consumed by the encoding.)

import re

def validate_fqdn(dn):
    if dn.endswith('.'):
        dn = dn[:-1]
    if len(dn) < 1 or len(dn) > 253:
        return False
    ldh_re = re.compile('^[a-z0-9]([a-z0-9-]{0,61}[a-z0-9])?$',
                        re.IGNORECASE)
    return all(ldh_re.match(x) for x in dn.split('.'))

One could argue for accepting empty domain names, or not, depending on one's purpose.

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

• contains at least one character and a maximum of 63 characters
• consists only of allowed characters
• doesn't begin or end with a hyphen.

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.