Parsing legal documents is hard. It's hard if you're a lawyer and it's even harder if you're a computer. Here's an article on the Free Law Project that describes the challenge of parsing legal documents: http://freelawproject.org/2013/11/01/want-to-merge-millions-of-legal-opinions-it-wont-be-easy/.

They have a collection of similarly great content on other practical problems that arise when parsing the law.