# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/(?:https?:\/\/)?(?:www\.)?\S+\.\S+$//gm;t' <<< "www.website1.com
www.website2.com
www.website3.nl
www.website3.nl !
www.website3.nl !
www.website3.nl !
www.website1.com
websit.ea.pl
websit.ea.pl
websit.ea.pl
http://www.website2.com
https://www.website2.com
"
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html