Some people have mentioned parser generators, but so far nobody has mentioned Menhir (http://gallium.inria.fr/~fpottier/menhir/).

It's an LR(1) parser generator for OCaml and Coq with a lot of extremely interesting features, such as genuinely good debugging support for grammars and the ability of generating error messages by example.

What this means is that after you write down your grammar, Menhir will give you examples of all the possible syntax errors that could occur. You can then write error messages for each case and get a parser with built-in error reporting for syntax errors.

This works a lot better than you'd think and I really wonder why nobody else implements this feature. Or for that matter, why they're not advertising it on the webpage! If you want to know more, look in the manual, section 11.