Athanasius Kircher, The Tower of Babel, 1679.

Babel Bridge

Create parsers with ease 100% in Ruby.


It's a gem, installed like any other:
sudo gem install babel_bridge

A Quick Example

To create a new parser, declare a new class and inherit from BabelBridge::Parser. Inside your parser class, list all your parsing rules.

To use your parser, create a new instance and call parse, passing in the string you wish to parse.

require "babel_bridge"

class MyParser < BabelBridge::Parser

  rule :foo, "foo", :bar?
  # rule named :foo
  # match the string "foo"
  # optionally followed by the rule :bar

  rule :bar, "bar"
  # rule named :bar
  # match "bar"


parser =
parser.parse "foo"     # matches "foo"
parser.parse "foobar"  # matches "foobar"



I was inspired by Treetop's use of parsing expression grammars in Ruby. PEGs let you think about parsing as a giant regular expression with function calls (sub-rules). However I found Treetop's custom .treetop files awkward to work with.

I wanted a way to express PEGs directly in my Ruby code. As an active Rails developer, I wanted to embrace Convention-Over-Configuration and DRY and come up with a declarative API similar to ActiveRecord. I started out writing patches to Treetop, but it quickly became evident my direction was incompatible with the existing codebase. So, on a whim, I started Babel Bridge. A few weeks later, and surprisingly little code (786 lines in 0.1.1), I am quite please with the result.

Overall, my goal is to make it easy to write grammars and easy to debug them when they aren't working.


The initial release is stable, but has only been lightly tested. Please let me know if you have problems.

System Requirements

Ruby 1.8 and 1.9 are supported.

Related Work

Other Ruby based parsers using parsing expression grammars:

Further reading

Project Links