find . -type f -name "*.htm" -print|xargs perl -i -pe 's/http:\/\/example1.com/[404 Not Found]/g'; find . -type f -name "*.htm" -print|xargs perl -i -pe 's/http:\/\/example2.com/[404 Not Found]/g'; find . -type f -name "*.htm" -print|xargs perl -i -pe 's/http:\/\/example3.com/[404 Not Found]/g'; etc.