public function normalizeKey($key)
	{
		// Built-in/custom filter, normalized to lowercase
		if (preg_match('/^#[a-z_0-9]+$/Di', $key))
		{
			return strtolower($key);
		}

		// Valid callback
		if (is_string($key) && is_callable($key))
		{
			return $key;
		}

		throw new InvalidArgumentException("Invalid filter name '" . $key . "'");
	}