It is for lesser mortals, too. I think everyone with half a CS degree should be able to pull it off.

That said: it's an implementation which doesn't behave like normal regexps (it's a left-shortest match first instead of the common left-longest) and it's a back-tracker. Because this implementation doesn't allow much choice, expensive examples will look contrived, but extending this implementation with character sets, groups and choices (i.e. a|b) will make its exponential nature felt very quickly.

It's a natural approach to anyone familiar with the pattern matchers explained in old Lisp books. This is not just hindsight because when I first read this chapter in Beautiful Code I stopped after the problem statement and wrote something similar before reading the rest.

(I didn't choose exactly the same grep sublanguage to implement -- it was something like supporting the most basic escaping instead of ^ and $.)