sed remove first two octets from ip

Question

I have a file that contains a bunch of ip addresses. I would like for my sed command to remove the first two octets of these. ex. 172.0.0.1 should be changed to XXX.ZZZ.0.1

What I have now remove the lasts octets and replace it with an x!

sed -i 's/\\(\\([0-9]\\{1,3\\}\\.\\)\\{3\\}\\)[0-9]\\{1,3\\}/\\1XXX/g' "${file}"

This is run in a bash scripts that takes the file containing the ip addresses as input param.

I haven't been able to figure out how to do this with sed yet so I'll appreciate any help.

Answer 1

It seems you may use

sed -i.bak -E 's/\b([0-9]{1,3}\.){2}([0-9]{1,3}\.[0-9]{1,3})\b/XXX.ZZZ.\2/g' "${file}"

The -E (or -r on other OSes) enables the POSIX ERE syntax that allows using fewer escapes in the pattern (no need to escape grouping ( and ) symbols and the limiting/range quantifier {n,m} ).

Details

• \\b - word boundary
• ([0-9]{1,3}\\.){2} - two occurences of (Group 1) one to three digits and a dot
• ([0-9]{1,3}\\.[0-9]{1,3}) - (Group 2) one to three digits, a . and again 1 to 3 digits
• \\b - word boundary

The whole match is replaced with Group 2 value ( \\2 ). The -i.bak will make the replacements in file, and an original copy with a .bak extension will be generated.

See the online demo .