Can't found how to write correct regexp for Symfony 4 @Route

Question

I'd try to limit route's url to hexadecimal characters, so written in Controller such code:

/**
 * @Route("/out_import/{nr}", name="out_import",
 * defaults={"nr"="empty"},
 * requirements={"nr"="\^[0-9A-Fa-f]{15}"}
 * )
 */
public function out_import($nr)

But it's not work, just bind any correct urls. Combination like \w+ or \w{15} not fit, because accept "_" etc. Please, help!