JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="foo" value="" />
<input type="button" onclick="testreg()" value='test' />

JavaScript

function testreg(){
chaine=document.getElementById('foo').value;
    alert( chaine.match(/^[0-9]{8}$|^$/)?"ok":"KO")


}