string_entities.rb

require 'htmlentities'

class String
  def plain_text
    coder = HTMLEntities.new
    coder.decode(self).gsub(/<\/?[^>]*>/, '')
  end

  def html
    coder = HTMLEntities.new
    self.gsub(/[^&><]/) { |match| coder.encode(match, :named) }
  end
end