Question

What is the fastest way to check if a string matches a certain pattern? Is regex the best way?

For example, I have a bunch of strings and want to check each one to see if they are a valid IP address (valid in this case meaning correct format), is the fastest way to do this using regex? Or is there something faster with like string formatting or something.

Something like this is what I have been doing so far:

for st in strs:
    if re.match('\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}', st) != None:
       print 'IP!'

Answer 1

Update

The original answer bellow is good for 2011, but since 2012, one is likely better using Python’s ipaddress stdlib module – besides checking IP validity for IPv4 and IPv6, it can do a lot of other things as well.

---

It looks like you are trying to validate IP addresses. A regular expression is probably not the best tool for this.

If you want to accept all valid IP addresses (including some addresses that you probably didn’t even know were valid) then you can use IPy (Source):

from IPy import IP
IP('127.0.0.1')

If the IP address is invalid it will throw an exception.

Or you could use socket (Source):

import socket
try:
    socket.inet_aton(addr)
    # legal
except socket.error:
    # Not legal

If you really want to only match IPv4 with 4 decimal parts then you can split on dot and test that each part is an integer between 0 and 255.

def validate_ip(s):
    a = s.split('.')
    if len(a) != 4:
        return False
    for x in a:
        if not x.isdigit():
            return False
        i = int(x)
        if i < 0 or i > 255:
            return False
    return True

Note that your regular expression doesn’t do this extra check. It would accept 999.999.999.999 as a valid address.