How do you extract IP addresses from files using a regex in a linux shell?

Most of the examples here will match on 999.999.999.999 which is not technically a valid IP address.

The following will match on only valid IP addresses (including network and broadcast addresses).

grep -E -o '(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)' file.txt

Omit the -o if you want to see the entire line that matched.

You could use grep to pull them out.

grep -o '[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}' file.txt