JSFiddle - React, Tailwind, and code Playground

HTML

<form name="myform">
    <input name="CreditCardNumber" />
</form>

JavaScript

function validate_creditcardnumber()
{

var re16digit=/^\d{16}$/
if (document.myform.CreditCardNumber.value.search(re16digit)==-1)
alert("Please enter your 16 digit credit card numbers");
return false;

}

document.myform.CreditCardNumber.onchange = validate_creditcardnumber;