To be fair, while it's not a panacea, Parse::RecDescent is actually a pretty good tool for exactly this kind of job. When I recently tried a similar exercise myself, my first readthrough of the manual left me sceptical, but in practice I was pleasantly surprised at how easy it turned out to be to parse expressions of this precise sort with Parse:RecDescent. Whereas I've never managed to get my head round LALR(1) parsing of the sort that Limbic~Region found so intuitive.

Though I suppose that just reinforces the point that one shouldn't assume that a tool is the right tool for one's own project just because other people prefer it...

(I do find myself wondering whether the love affair with Yapp would have continued beyond the first shift/reduce conflict...)