parslet is a lot easier to work with

bbsandbox.rb

@@ -3,8 +3,9 @@ require 'slim'
 require 'digest/sha1'
 require 'bb-ruby'
 require 'ruby-bbcode'
+require './bristlecode.rb'
 
-@@engines = ['bb-ruby', 'ruby-bbcode', 'raw']
+@@engines = ['bb-ruby', 'ruby-bbcode', 'bristle', 'raw']
 
 get '/' do
   @posts = list_posts
@@ -35,6 +36,8 @@ def exec_bbcode(engine, body)
     BBRuby.to_html body
   when "ruby-bbcode"
     RubyBBCode.to_html body
+  when "bristle"
+    Bristlecode::to_html body
   when "raw"
     body
   else

bristlecode.rb

@@ -0,0 +1,109 @@
+require 'parslet'
+
+module Bristlecode
+
+  def Bristlecode.to_html(text)
+    parser = Bristlecode::Parser.new
+    parse_tree = parser.parse(text)
+    tree = Bristlecode::Transform.new.apply(parse_tree)
+    tree.to_html
+  end
+
+  class Parser < Parslet::Parser
+    rule(:space) { match('\s').repeat(1) }
+    rule(:space?) { space.maybe }
+
+    rule(:bold_open) {
+      (str('[b]') | str('[B]'))
+    }
+    rule(:bold_close) {
+      (str('[/b]') | str('[/B]'))
+    }
+    rule(:bold) {
+      (bold_open >> children.as(:bold) >> bold_close)
+    }
+
+    rule(:italic_open) {
+      (str('[i]') | str('[I]'))
+    }
+    rule(:italic_close) {
+      (str('[/i]') | str('[/I]'))
+    }
+    rule(:italic) {
+      (italic_open >> children.as(:italic) >> italic_close)
+    }
+
+    rule(:eof) { any.absent? }
+    rule(:tag) { (bold | italic) }
+    rule(:elem) { text | tag }
+    rule(:tag_open) { bold_open | italic_open }
+    rule(:tag_close) { bold_close | italic_close }
+    rule(:tag_delim) { tag_open | tag_close }
+
+    rule(:text) { (tag_delim.absent? >> any).repeat(1).as(:text) }
+    rule(:children) {
+      space? >> elem.repeat(1)
+    }
+    rule(:doc) {
+      space? >> elem.repeat(1).as(:doc)
+    }
+    root(:doc)
+  end
+
+  class Transform < Parslet::Transform
+    rule(bold: sequence(:children)) { Bold.new(children) }
+    rule(italic: sequence(:children)) { Italic.new(children) }
+    rule(text: simple(:text)) { Text.new(text) }
+    rule(doc: subtree(:doc)) { Doc.new(doc) }
+  end
+
+  class Doc
+    def initialize(children)
+      @children = children
+    end
+
+    def to_html
+      s = StringIO.new
+      @children.each{|child| s << child.to_html }
+      s.string
+    end
+  end
+
+  class Text
+    def initialize(text)
+      @text = text
+    end
+
+    def to_html
+      @text
+    end
+  end
+
+  class Bold
+    def initialize(children)    
+      @children = children
+    end
+
+    def to_html
+      s = StringIO.new
+      s << "<b>"
+      @children.each{|child| s << child.to_html }
+      s << "</b>"
+      s.string
+    end
+  end
+
+  class Italic
+    def initialize(children)
+      @children = children
+    end
+
+    def to_html
+      s = StringIO.new
+      s << "<i>"
+      @children.each{|child| s << child.to_html }
+      s << "</i>"
+      s.string
+    end
+  end
+end