LEX/YACC

Have any of you ever used LEX/YACC? If so, what did you use it for?

I used it to parse cue files.

I wanted to write an ed clone, so I made a grammar for the commands. Unfortunately it was in Perl (Parse::Yapp), which as I found out it isn't suitable for anything slightly complicated.

Currently I think monadic parser combinators are a better idea than a not-even-Touring tarpit.