Flex and Bison come to mind, but they are tailored more for the point of generating new language parsers and compilers. You could, however, attempt to use them

Flex and Bison come to mind, but they are tailored more for the point of generating new language parsers and compilers. You could, however, attempt to use them in another fashion, considering they are open source (BSDish and GPL respectively). Although I have looked, I've been unable to find much more than a bunch of whitepapers and various incomplete projects.