Well, you can consider it a bit of a litmus test. If you can't figure out how to use these programs, you're probably not up to making a compiler, or even an interpreter for another language.

By this time you should also know that O'Reilly makes some of the best technical books, especially when it come to programming languages, and they actually have a book on Lex and YACC. The GNU versions naturally, but it's probably as good a starting point as any.