Minimal Ruby S-expression parser w/test case showing using %(string syntax) to naturally embed s-expressions in Ruby code.

sexp.rb

require 'strscan'
class Sexp
  def initialize(str); @s = StringScanner.new(str); end
  def s(e); @s.scan(e); end
  def r(e); s(e) or raise "Expected #{x}"; end
  def list; s("(")&&b=body and r(")") and b; end
  def exp; s(/[a-zA-Z][\w_-]*/)&.to_sym||s(/[0-9]+/)&.to_i||s(/"[^"]*"/)&.[](1..-2)||list||r("exp"); end
  def body; r=[]; while s(/\s*/) and !@s.eos? and !@s.check(")") and e=exp; r<<e; end; r; end
end
def sexp str; Sexp.new(str).body; end

p sexp %(
  this (is a (test) 42 "some str")
)