Replace newlines with literal \n

Question

This stackoverflow question has an answer to replace newlines with sed, using the format sed \':a;N;$!ba;s/\\n/ /g\'.

This works, but not for special characters like

Answer 1

This should work with both LF or CR-LF line endings:

sed -E ':a;N;$!ba;s/\r{0,1}\n/\\n/g' file