JSFiddle - React, Tailwind, and code Playground

HTML

<textarea name="comment">Lorem ipsum dolor https://www.youtube.com/embed/mr7FXvTSYpA lorem ipsum dolor</textarea>

<div class="output"></div>

CSS

textarea {
  width: 100%;
}

JavaScript

var comment = document.querySelector('textarea');
var output = document.querySelector('div');

var str = comment.value;
var regex = /(https:\/\/www.youtube.com\/embed\/\w*)/;

if (regex.test(str)) {

  str = str.replace(regex, '<iframe width="560" height="315" src="$1" frameborder="0" allowfullscreen></iframe>');
  output.innerHTML = str;

}