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

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 e

Answer 1

For those who want a ready solution for getting IP addresses from apache log and listing occurences of how many times IP address has visited website, use this line:

grep -Eo '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}' error.log | sort | uniq -c | sort -nr > occurences.txt

Nice method to ban hackers. Next you can:

1. Delete lines with less than 20 visits
2. Using regexp cut till single space so you will have only IP addresses
3. Using regexp cut 1-3 last numbers of IP addresses so you will have only network addresses
4. Add deny from and a space at the beginning of each line
5. Put the result file as .htaccess