python - Regexp to check if an IP is valid

Question

I'm wondering if it's possible to compare values in regexps with the regexp system in Python. Matching the pattern of an IP is easy, but each 1-3 digits cannot be above 255 and that's where I'm a bit stumped.

Answer 1

No need for regular expressions here. Some background:

>>> import socket
>>> socket.inet_aton('255.255.255.255')
'xffxffxffxff'
>>> socket.inet_aton('255.255.255.256')
Traceback (most recent call last):
  File "<input>", line 1, in <module>
error: illegal IP address string passed to inet_aton
>>> socket.inet_aton('my name is nobody')
Traceback (most recent call last):
  File "<input>", line 1, in <module>
error: illegal IP address string passed to inet_aton

So:

import socket

def ip_address_is_valid(address):
    try: socket.inet_aton(address)
    except socket.error: return False
    else: return True

Note that addresses like '127.1' could be acceptable on your machine (there are systems, including MS Windows and Linux, where missing octets are interpreted as zero, so '127.1' is equivalent to '127.0.0.1', and '10.1.4' is equivalent to '10.1.0.4'). Should you require that there are always 4 octets, change the last line from:

else: return True

into:

else: return address.count('.') == 3