private function convert($callback, $variables = []) {

		$this -> number = preg_replace_callback('~[0-9\.,]+~', function($number) use ($callback, $variables) {
			return call_user_func_array($callback, array_merge([(float) str_replace(',', '', $number[0])], $variables));
		}, $this -> number);

		return $this;
	}