Parsing for Fun and Profit (slides & code)

Here are the slides and code for my talk Parsing for Fun and Profit given at North West Ruby User Group (Manchester, UK) in February 2013. Slides first, then a guided tour of the code at the bottom of this post.


Talk summary

The talk is a very brief overview of what parsing is, why you'd want to do it and what you can do with a parser. The goal is not to go into detail about parsing or any of the tools used, or to show any "parsing best practices", just to show that building a simple language application is more accessible than you might think.

The specific examples are done by writing Parsing Expression Grammars in Treeptop. I show how to build a grammar one rule at a time by incrementally building up a suite of examples in RSpec, taking the Arithmetic sample grammar from the Treetop gem. As an example of a more complete language application with a more complex grammar, I show how to build a syntax highlighter for a simple subset of Ruby, which turns source code into marked-up HTML.

Code highlights

The source code is availble at You may like to browse all the example source, but here are some highlights:

  • arithmetic_parser_spec.rb is the example we worked through semi-live, and shows how you can build a grammar one rule at a time
  • simple_ruby.treetop is a Treetop grammar for a very small subset of Ruby - it's by no means production quality, but it's expressive enough for our demo purposes
  • simple_ruby_parser_spec.rb shows how you can build a complex grammar by inspecting a simplified version of the parse tree that Treetop generates
  • simple_ruby_parser.rb contains the code that generates these simplified syntax trees
  • spec_helper.rb shows how to get helpful error messages from Treetop - it relies on the tree simplifier in the SimpleRubyParser node classes
  • bin/rb2html is our little syntax highlighter application - it only takes about 20 lines of code!

Most of the code is commented to explain why it's done the way it is.


If you have any questions or comments, feel free to contact me at