How to replace link part with html css js?

Question

css

It is required to make an input line into which a specific link (any) fits, for example : " https://htmlforum.org/forum/123 "  //htmlforum.org/forum/123 "  It became " https://123htmlsuper.ru/forum/123 " That is, not just from org to ru, but completely from https to a slash before org With any link, not just this one , there is one solution, but ran into an error.

<input id="input" type="text" value="https://htmlforum.io/topic/63758-zamena-teksta-na-html-css-js/">
<br>
<button id="button">Update URL</button>
<br>
<div id="result"></div>

document.getElementById("button").addEventListener("click", function() {

  var input = document.getElementById('input');
  var result = document.getElementById('result');
  var url = input.value;
  var index = url.search(/[\/\/]\w*.(\/)/gm);

  var newDomain = "https://html.com";
  var newUrl = newDomain + url.substr(index);

  result.innerText = newUrl;
  
});

search(/[\/\/]\w*.(\/)/gm) throws an error at this point
Uncaught typeerror:cannot read property 'search' of undefined at HTMLbuttonElement. (1.html:11)