h.rb

#!/usr/bin/env ruby
$:.unshift(".")
require "treetop"
require "huml"
require "pp"

class Huml::MultipleID < Exception; end
class Huml::MultipleName < Exception; end

pp HumlParser.new.parse("%html").cons
pp HumlParser.new.parse("%html.foo").cons
pp HumlParser.new.parse("%html#foo").cons
pp HumlParser.new.parse("%html.foo.bar#head").cons
pp HumlParser.new.parse(".foo").cons
pp HumlParser.new.parse("#foo").cons
pp HumlParser.new.parse("#foo#bar").cons     # raise Exception
pp HumlParser.new.parse("%foo%bar").cons     # raise Exception

huml.treetop

grammar Huml
    rule top
        ( tag ) {
            def cons
                elements.cons
            end
        }
    end
    rule tag
        name classes id {
            def cons
                name.cons + classes.cons + id.cons
            end
        }
    end
    rule name
        ("%" [a-zA-Z]+)* {
            def cons
                raise Huml::MultipleName if elements.length > 1
                [:name, (text_value.empty? ? "div" : elements.first.text_value)]
            end
        }
    end
    rule classes
        ("." item:[a-z_-]+)* {
            def cons
                elements.length > 0 ? [:class, elements.map(&:item).map(&:text_value)] : []
            end
        }
    end
    rule id
        ("#" [a-z_-]+)* {
            def cons
                raise Huml::MultipleID if elements.length > 1
                elements.first ? [:id, elements.first.text_value] : []
            end
        }
    end
end