Another nice thing about ed is that it is sometimes easier to use than sed when you want to edit a file programmatically, since you can navigate lines at random (forward and backward directions), and you can still run regex find/replace like with sed. Just

printf 'i\nstring of ed commands\n.\n' | ed file-to-edit.txt

and pipe the commands into ed, although it is really an esoteric way to write scripts.