[SOLVED] Unable to exclude IPv4 addresses using regex in grep

Issue

I used a regex to grep and output only IPv4 addresses from the file content.

But when I try to use the same regex to exclude all IPv4 addresses, it just does not work.

File content:

# cat IPs
172.16.1.125
172.16.1.4
172.16.1.143
172.16.1.140
172.16.1.77
/dev/nvme101
/dev/sda1
/dev/sdb2
172.16.1.60
172.16.1.146
172.16.1.5
172.16.1.51
172.16.1.99
172.16.1.10
172.16.1.189

To grep only IPv4 addresses:

# grep -oE "\b([0-9]{1,3}\.){3}[0-9]{1,3}\b" IPs
172.16.1.125
172.16.1.4
172.16.1.143
172.16.1.140
172.16.1.77
172.16.1.60
172.16.1.146
172.16.1.5
172.16.1.51
172.16.1.99
172.16.1.10
172.16.1.189

When I try to exclude the IPv4 addresses using the same regex:

# grep -voE "\b([0-9]{1,3}\.){3}[0-9]{1,3}\b" IPs
#

No output at all.

I was expecting the following output:

/dev/nvme101
/dev/sda1
/dev/sdb2

Solution

Get rid of the -o. The -o flag says to only show what was matched rather than the entire line. That doesn't make sense when using -v for lines that do NOT match.

In ack, if you try to use -o and -v together, it throws an error.

Answered By - Andy Lester

Answer Checked By - Katrina (WPSolving Volunteer)