# Links


The primary resource for all things to do with parsing expressions can be found
on the original [Packrat and Parsing Expression Grammars page](http://pdos.csail.mit.edu/~baford/packrat) at MIT.

Also, a useful summary of parsing expression grammars can be found on 
[Wikipedia](http://en.wikipedia.org/wiki/Parsing_expression_grammar).

Citrus draws inspiration from another Ruby library for writing parsing
expression grammars, Treetop. While Citrus' syntax is similar to that of
[Treetop](http://treetop.rubyforge.org), it's not identical. The link is 
included here for those who may wish toexplore an alternative implementation.