An example grammar/parser using the Ruby gem Treetop

my_parser.rb

require 'treetop'

module Mine

  class IntegerLiteral < Treetop::Runtime::SyntaxNode
    def to_i
      text_value.to_i
    end
  end

  class Expression < Treetop::Runtime::SyntaxNode
    def value
      elements.map(&:to_i).reduce(:+)
    end
  end

  class Parser

    def initialize
      Treetop.load_from_string(DATA.read)
      @parser = MineParser.new
    end

    def parse(parse_string)
      tree = @parser.parse(parse_string)
      raise Exception, @parser.failure_reason if tree.nil?
      clean_tree!(tree)
      tree
    end

    private

    def clean_tree!(root_node)
      return if root_node.elements.nil?
      root_node.elements.delete_if { |node| node.class.name == "Treetop::Runtime::SyntaxNode" }
      root_node.elements.each { |node| clean_tree!(node) }
    end

  end

end

tree = Mine::Parser.new.parse('5 + 10')
puts tree.inspect; puts tree.value

__END__

grammar Mine

  rule expression
    integer space? '+' space? integer <Expression>
  end

  rule integer
    [0-9]+ <IntegerLiteral>
  end

  rule space
    [\s]+
  end

end