This tutorial is brilliant. It explains the usage of lex and yacc very clearly. The second part of this video tutorial is here:

https://www.youtube.com/watch?v=__-wUHG2rfM

The source code for this tutorial can be downloaded from GitHub:

Part1: https://github.com/jengelsma/lex-tutorial

Part2: https://github.com/jengelsma/yacc-tutorial