gistfile1.rb

# bbcode.treetop
grammar BBCode
  rule bbcode_post  # [b]hey[/b]
    opening_tag enclosed_text closing_tag
  end
  rule enclosed_text # hey
    [^\[]+ # Match until '['
  end
  rule code  # b
    [^\]]+ # Match until ']'
  end
  rule opening_tag  # [b]
    '[' code ']'
  end
  rule closing_tag  #[/b]
    '[/' code ']'
  end
end

parse_bbcode.rb

require "treetop"
Treetop.load "bbcode"

parser = BBCodeParser.new
text = "[b]hey[/b]"

tree = parser.parse(text)
puts tree.opening_tag.text_value    #=> [b]
puts tree.enclosed_text.text_value  #=> hey
puts tree.closing_tag.text_value    #=> [/b]