protected function _getCacheId($prefix = 'minify')
    {
        $name = preg_replace('/[^a-zA-Z0-9\\.=_,]/', '', $this->selectionId);
        $name = preg_replace('/\\.+/', '.', $name);
        $name = substr($name, 0, 100 - 34 - strlen($prefix));
        $md5 = md5(serialize(array(
            Minify_SourceSet::getDigest($this->sources),
            $this->options['minifiers'],
            $this->options['minifierOptions'],
            $this->options['postprocessor'],
            $this->options['bubbleCssImports'],
            Minify::VERSION,
        )));

        return "{$prefix}_{$name}_{$md5}";
    }