JSFiddle - React, Tailwind, and code Playground

HTML

<div id="matches"></div>

JavaScript

//var _reg = /^((?:[a-z\d]+(?:.(?!$))?)+)$/i;
var _reg = /^(?!=.*?\.\.)[a-z\d][a-z\d.]{4}[a-z\d]$/i;

var _tests = new Array();
_tests[0] = "jd1.2f";
_tests[1] = "j....f";
_tests[2] = "ek.KL1";
_tests[3] = "ekk.112";

for (var i = 0; i < _tests.length; i++) {
    var _matches = _tests[i].match(_reg);

    if (_matches && _matches.length > 0) $('#matches').append(_matches[0] + '<br/>');
}