I have a regexp, it works fine in Chrome/Firefox, but get null in Edge.
var regexp = new RegExp("<header.*(?=)(.|\n)*?</header>", "g");
var str = "<body><header><h2>Title</h2></header><footer>footer</footer></body>";
var result = regexp.exec(str);
console.log(result[0]);
JSFiddle demo: https://jsfiddle.net/JSDavi/k391qs72/2/
Please open it in different browsers. It's weird in Edge.