Colin Shivers with a discussion about the usual 80% solutions versus the benefit of a 100% solution, followed by his 100% solution of regexes represented by S-expressions for scsh (using Henry Spencer's famous Posix regexp engine in its core):

https://www.khoury.northeastern.edu/~shivers/papers/sre.txt

I see no source. Web searching also fails, although I see a re-implementation in Racket, "irRegular expressions":

https://docs.racket-lang.org/irregex/index.html

And this paper coauthored by Colin Shivers: "trx: Regular-tree expressions, now in Scheme" https://www.khoury.northeastern.edu/home/shivers/papers/trx.pdf

Oh, well, I suppose the code would have bit-rotted by now anyway.