JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="myInput" />
<input type="button" id="btn" value="ciao" />

JavaScript

var btn = document.getElementById('btn');
btn.onclick = function () {
    var check = true;
    var txt = document.getElementById('myInput');
    tmp = txt.value;
    if (tmp && tmp.length > 0) {

        arg = tmp.split('.');
        if (arg && arg.length > 1) {

            check = arg[0].length <= 6;
            if (arg.length > 1 && check) {
                check = arg[1].length <= 2;
            }
        }
        if (arg && arg.length > 0) {

            check = arg[0].length <= 6;
           
        }


    }
    if (!check) alert('check failed');

}