private function validate($content, $locale)
    {
        $validation = $this->markupParser->validate($content, $locale);

        $regex = sprintf(self::INVALID_REGEX, $this->markupNamespace, $this->markupNamespace);
        foreach ($validation as $tag => $state) {
            if (false === strpos($tag, 'sulu-validation-state="' . $state . '"')) {
                $newTag = preg_replace($regex, '$1 sulu-validation-state="' . $state . '"$2', $tag);
                $content = str_replace($tag, $newTag, $content);
            }
        }

        return $content;
    }