ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
  error_class = "with_errors"
  if html_tag =~ /<label/
    error_message = instance.object.errors.on(instance.method_name)
    html_tag.sub! /<\/label>/, " <em class=\"#{error_class}\">#{error_message}</em></label>"
  elsif html_tag =~ /<(input|textarea|select)[^>]+class=/
    class_attribute = html_tag =~ /class=['"]/
    html_tag.insert(class_attribute + 7, "#{error_class} ")
  elsif html_tag =~ /<(input|textarea|select)/
    first_whitespace = html_tag =~ /\s/
    html_tag[first_whitespace] = " class=\"#{error_class}\" "
  end
  html_tag
end