JSFiddle - React, Tailwind, and code Playground

HTML

<div id="alertEmail"></div>
<input id="emailinput" type="text" />
<button id="check">check<button>

JavaScript

function CheckEmail(email)
{
//regular expression to check the email field
var expr = /^([0-9a-zA-Z]+([_.-]?[0-9a-zA-Z]+)*@[0-9a-zA-Z]+[0-9,a-z,A-Z,.,-]*(.){1}[a-zA-Z]{2,4})+$/;

if (expr.test(email)) {
    document.getElementById('alertEmail').innerHTML = '<img title="Valid Email" src="images/icoOk.jpg" alt="Ok"/>';
    f = true;
    return true;
}
else {
    document.getElementById('alertEmail').innerHTML = '<img title="Invalid Email!" src="images/icoErro.gif" alt="Error"/>';
    f = false;
    return false;
}
}
document.getElementById('check').addEventListener('click', function(e){
    CheckEmail(document.getElementById('emailinput').value);
}, false);