Question: How to extract a text part by regexp in linux shell? Lets say, I have a file where in every line is an IP address, but on a different position. What is the simplest way to extract those IP addresses using common unix command-line tools?

Answer: 
You could use awk, as well. Something like ...

```
awk '{i=1; if (NF > 0) do {if ($i ~ /regexp/) print $i; i++;} while (i <= NF);}' file

```

May require cleaning. just a quick and dirty response to shows basically how to do it with awk.
```
cat ip_address.txt | grep '^[0-9]\{1,3\}[.][0-9]\{1,3\}[.][0-9]\{1,3\}[.][0-9]\{1,3\}[,].*$\|^.*[,][0-9]\{1,3\}[.][0-9]\{1,3\}[.][0-9]\{1,3\}[.][0-9]\{1,3\}[,].*$\|^.*[,][0-9]\{1,3\}[.][0-9]\{1,3\}[.][0-9]\{1,3\}[.][0-9]\{1,3\}$'

```

Lets assume the file is comma delimited and the position of ip address in the beginning ,end and somewhere in the middle

First regexp looks for the exact match of ip address in the beginning of the line.
The second regexp after the or looks for ip address in the middle.we are matching it in such a way that the number that follows ,should be exactly 1 to 3 digits .falsy ips like 12345.12.34.1 can be excluded in this.

The third regexp looks for the ip address at the end of the line