Convert CIDR notation to regex

cidr2regex.py

#!/usr/bin/python
''' Not my script, found on the Internet, and rediscovered on my hard drive
'''
import sys

def cidr_to_regex(cidr):
    ip, prefix = cidr.split('/')

    base = 0
    for val in map(int, ip.split('.')):
        base = (base << 8) | val

    shift = 32 - int(prefix)
    start = base >> shift << shift
    end = start | (1 << shift) - 1

    def regex(lower, upper):
        if lower == upper:
            return str(lower)

        from math import log10
        exp = int(log10(upper - lower))
        delta = 10 ** exp

        if lower == 0 and upper == 255:
            return "\d+"

        if delta == 1:
            val = ""
            for a, b in zip(str(lower), str(upper)):
                if a == b:
                    val += str(a)
                elif (a, b) == ("0", "9"):
                    val += '\d'
                elif int(b) - int(a) == 1:
                    val += '[%s%s]' % (a, b)
                else:
                    val += '[%s-%s]' % (a, b)
            return val

        def gen_classes():
            floor_ = lambda x: int(round(x / delta, 0) * delta)

            xs = range(floor_(upper) - delta, floor_(lower), -delta)
            for x in map(str, xs):
                yield '%s%s' % (x[:-exp], r'\d' * exp)

            yield regex(lower, floor_(lower) + (delta - 1))
            yield regex(floor_(upper), upper)

        return '|'.join(gen_classes())

    def get_parts():
        for x in range(24, -1, -8):
            yield regex(start >> x & 255, end >> x & 255)

    return '^%s$' % r'\.'.join(get_parts())

for line in sys.stdin.readlines():
	print cidr_to_regex( line )
	print

Thanks for rediscovering this, I owe you a beer should we ever meet

Thanks a lot!

Just some bugs:

1.2.3.8/29   -> 1.2.3.8 - 1.2.3.15
regex: 1\.2\.3\.[1-8]

172.1.8.0/22 -> 172.1.8.0 - 172.1.11.255
regex :  172\.1\.[1-8]\.\d+

Hi, total Python noob here. How do I enter the CIDR so that I can test this script?

are you in linux?

python2 thescript.py
172.1.8.0/22d

d sends an end of file to window/process/ect

another issue. try cidr 204.16.248.0/22. i'm seeing ^204.16.2[45][8-1].\d+$
on centos and ubuntu, which misses all of 204.16.249.0/24.

i think this line should be

https://gist.github.com/petermblair/2294250#file-cidr2regex-py-L51

return '(' + '|'.join(gen_classes()) + ')'

I suggest checking out this derived version:

https://gist.github.com/tom-knight/1b5e0dcf39062af8910e

I dug down through all the forks and found this one to be the most promising. Hope this saves other people time!

Javascript version: https://gist.github.com/radu-c/df0408cb55de758e03b7568cc05f1865