gistfile1.rb

ALLOW_TAGS = %w/font span p b br/
def validate_xml_fragment
  str = self
  tags = str.scan(/\G(?:(<[^<>]+>)|[^<>&]+|[^&<>]*(?:&#?\w+;[^&<>]*)+)/)
  return ERB::Util.html_escape(str) unless $'.empty?
  stack = []
  tags.each do |tag|
    case tag.first
    when /<(\w+)(?: +\w+=(?:"[^"]*"|'[^']*'|\w+))* *(\/)?>/
      return ERB::Util.html_escape(str) unless ALLOW_TAGS.include?($1)
      stack.push($1) unless $2
    when /<\/(\w+)>/
      return ERB::Util.html_escape(str) unless stack.last == $1
      stack.pop
    when nil
    else
      return ERB::Util.html_escape(str)
    end
  end
  return ERB::Util.html_escape(str) unless stack.empty?
  nil while str.gsub!(/^\s+|\s+$|\A<br *\/>|<br *\/>\z/, '')
  str
end