grep -Rli line20arpr.rr.nu ~/*
grep, of course, searches for a string in the listed files
-R will grep recursively
-l (lower case ell) will only list the files containing the results
-i will ignore case, so RR.nU will be found
line20arpr.rr.nu is the string you're searching for. Adding slashes, question marks, less-than and greater-than signs, quotes, etc. makes the command more complicated. I assume that line...nu doesn't exist naturally in your files, so it's simpler to just search for that.
~/* means all the files in your home directory. yourdomain.com is in a subdirectory of your home directory, so it will get searched.