As in a regex or ..?

An ipv4 parser would also be sorta difficult.

you have to account for the fact that all the octets can be added to decimal: http://2130706433 (valid 127.0.0.1)

or the fact that octets can be in different formats: http://0x7F.0x0.0x0.0x1 (127.0.0.1)

or the fact that you can mix octet formats: http://0xC0.0250.0.1 (192.168.0.1)