protected function _normalizeTag($tag) {
		$namespace = null;
		$label = $tag;
		$separator = $this->getConfig('separator');
		if ($separator === null) {
			return [
				null,
				$tag,
			];
		}

		if (strpos($tag, $separator) !== false) {
			list($namespace, $label) = explode($separator, $tag);
		}

		return [
			trim($namespace),
			trim($label)
		];
	}