JSFiddle - React, Tailwind, and code Playground

HTML

<form name="myform" onsubmit="return validate_creditcardnumber();">
<input id=CreditCardNumber maxlength=16>
    <input type="submit" value="submit">
</form>

JavaScript

function validate_creditcardnumber() {
    var re16digit = /^\d{16}$/
    if (!re16digit.test(document.myform.CreditCardNumber.value)) {
        alert("Please enter your 16 digit credit card numbers");
        return false;
    }
}