Remove the main domain from a list of subdomains

Question

How to remove the main domain from a list of subdomains for example if i have a file contains

www.example.com
sub.examle.com
sub2.example.com
examle.com.ar
example.com

I want to extract only

sub.example.com
sub2.example.com

I try grep -v '^www.example.com' file.txt how can i get sub domains

Answer 1

• First use Perl regular expression by using the P option.
• ^(?!www\.) get all the strings that do not begin with a www. ((?!www\.) negative lookahead).
• \w+\. matches any number of word characters (equal to [a-zA-Z0-9_]) followed by a period.
• \w{4,} match more than 4 word characters.

grep -P '^(?!www\.)\w+\.\w{4,}' file.txt
#> sub.examle.com
#> sub2.example.com