regex - Validating IPv4 addresses with regexp

Question

I've been trying to get an efficient regex for IPv4 validation, but without much luck. It seemed at one point I had had it with (25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?(.|$)){4}, but it produces some strange results:

$ grep --version
grep (GNU grep) 2.7
$ grep -E '(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?(.|$)){4}' <<< 192.168.1.1
192.168.1.1
$ grep -E '(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?(.|$)){4}' <<< 192.168.1.255
192.168.1.255
$ grep -E '(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?(.|$)){4}' <<< 192.168.255.255
$ grep -E '(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?(.|$)){4}' <<< 192.168.1.2555
192.168.1.2555

I did a search to see if this had already been asked and answered, but other answers appear to simply show how to determine 4 groups of 1-3 numbers, or do not work for me.

Answer 1

You've already got a working answer but just in case you are curious what was wrong with your original approach, the answer is that you need parentheses around your alternation otherwise the (.|$) is only required if the number is less than 200.

'((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(.|$)){4}'
    ^                                    ^