Converts any context-free grammar into a weakly equivalent grammar in Chomsky Normal Form

A grammar is in Chomsky Normal Form if all its productions are either of the form: A -> BC or of the form: A -> a

Any context-free grammar can be matched to some weakly equivalent grammar in CNF. Weakly equivalent grammars accept/reject the same inputs, but the parses they produce may not have the same structure.

Conversion to Chomsky Normal Form is useful for certain parsing algorithms.

The input file should be a context-free grammar in a format supported by NLTK

Example grammar file:

# my_grammar.cfg
S -> NP VP
NP -> Det N
VP -> V | V NP
Det -> 'the' | 'a'
N -> 'cat' | 'dog'
V -> 'runs' | 'chases' | 'catches' | 'eats'

Example usage:

python cnf.py my_grammar.cfg > my_cnf_grammar.cfg

This code was developed using Python 2.7 and NLTK 3.2