Validate a hostname string

import redef 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.

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 redef 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)

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