Recursive descent parser

parser.rb

# Recursive descent parser
# http://adayinthepit.com/2011/07/19/hanging-in-the-treetops/
def exp(stack)
  token = stack.shift
  return stack if token =~ /[0-9]/ || token =~ /X/ && exp(stack) || token =~ /Y|Z/ && exp(stack) && exp(stack)
  false
end

def parse(source)
  stack = exp(source.split(''))
  stack && stack.size == 0
end