[kps]

I used ed for a few months long ago, working with a mainframe Unix-emulation environment that didn't have character I/O working yet, only line-by-line from the host OS. It taught me how to use regular expressions effectively, since they were the only way to make changes within a line. I think I still do a greater fraction of editing with RE operations than most people.

I know there are Linux distributions now that don't even include ed by default, even though it's in POSIX and occasionally used in scripts.