Payer is a parser/parser generator library written in Python. It aims to provide a flexible framework for parsing regular and context free languages with varying levels of grammatical ambiguity, while allowing for modular grammars which are programmatically modifiable within Python. Payer consists of two parts: Matcher, a library which can be used to write pattern matching function definitions a la functional languages such as ML, Haskell, or Mathematica, and Payer, itself, which uses Matcher to implement pattern matching algorithms on symbolic representations of languages. For a simple usage example see samples/hello_world.py. Or read this instructive paper to get an idea of what derivative based parsing is all about: http://matt.might.net/papers/might2011derivatives.pdf