Perl6 is pretty awesome in that regard. The grammar specification is written in the language itself[0] in a single file using the language's rules[1] feature. Perl6 is an amazing language, but it is hampered by a runtime that is currently too slow, a bad public perception, and a tiny community.

[0]: https://github.com/perl6/std/blob/master/STD.pm6

[1]: http://en.wikipedia.org/wiki/Perl_6_rules