Perl grok's config grammar was super easy to write thanks to Parse::RecDescent. In the C++ version, I wanted similar ease. However, the tools I had available didn't appear to be expressive enough to support what I wanted. The config object in C++ was going to be a class, so you were free to have multiple config objects, and which meant I couldn't have any global variables. Both Boost Xpressive and Boost Spirit support grammar parsing almost trivially, but they require awkward wrapping and basically make it very hard to use when you want to update values in a class instance instead of some global variables.
Eventually, I gave up and wrote my own recursive descent bits using Xpressive to do the pattern matching and some trivial in-object state management to keep track of what was going on. It was really simple, despite my fears.
I'm not really sure what made me afraid of doing it, but the fear was totally unfounded.