How to substitute * in the middle of the card number?

Question

JavaScript

I want to write a credit card number mask, send it to the server with asterisks too. The problem is that val doesn't change

<input name="paymentCard" class="form-control" type="number" placeholder="**** **** **** ****" id="paymentCard">

function paymentCard() {
  if ($('#paymentCard').val() === "") {
    return '<?= lang("not_completed")?>';
  } else {
    const str = $('#paymentCard').val();
    const regex = /(?<=\d{4})\d(?=\d{4})/gm;
    const subst = `*`;
    return str.replace(regex, subst);
  }
}

$('#paymentCard').on('keyup', function () {
  paymentCard()
});



тут поклацать

what's wrong?