Given a field such as <input id="notes" type="text" names="notes"> You can detect a Credit Card number using the following in jQuery. This will detect 15 or 16 digit credit card numbers because of the d{3,4}. To only detect 15 or 16 change that to be only d{3} for 15 or d{4} for 16.

$('#notes').on('input', function() {
    const ccDetected = $('#notes').val().match(/\b(?:\d{4}[ -]?){3}(?=\d{3,4}\b)/gm);
    if (ccDetected) {
        alert('don''t use credit cards!');
    }
});