I can't recall what it's from. All the smart Linux geeks (of which group I do not claim to be a part of) I know are familiar with it. It might be a standard Linux command/tool which is combined with regex.
I've seen people use it for years in the IRC channel, and from context I learned to just think of it as saying, in the example of "s/this/that", "switch this for that." So I've used it from time to time as well, not knowing anything of its origin. I only recently learned it was from something specific, which as I said, I think is Linux-centric.
Edvard would know what it's from. I'm sure f0dder would, too.