If anybody is thinking about writing a parser in Ruby I can recommend the Parslet (https://github.com/kschiess/parslet) PEG parser library. I used it to construct a parser for a Lisp-like grammar and I found it robust and easy to use.

(You might not think you'd need a full blown parser library for something with a grammar as simple as Lisp but I found that there are enough grammar rules and the lookahead is deep enough that using a PEG parser was a win.)