Interesting that it's on paste.lisp.org. It strongly reminds me of Emacs' rx macro. I suspect this a port of another toy regexp implementation -- otherwise I would expect the author to use `itertools.chain` rather than implementing something that does exactly the same (`iconcat` in this sample).
It was, though it was fairly new at that time, so the author may not have had it on his system.
Also, the Python standard library is quite large. Most people will never really know about all its nooks and crannies, and it's easy to forget they exist. I've accidentally reimplemented parts of it more than once.
