JSFiddle - React, Tailwind, and code Playground

HTML

<div id="msg">Hi, this is a Textnode _link_<br>
this is Textnode number 2
   <br /><a href="http://abhi.cc">element node</a></div>
<br><br>
<br>
<br>
<br>
<b>This is how the output should be:</b><br>
-----------------------------------------------
<br>
Hi, this is a Textnode <a href="http://google.com">Google</a><br>
this is Textnode number 2
   <br /><a href="http://abhi.cc">element node</a></div>

CSS

* { font:14px/22px Tahoma,Arial,Sans-serif }

JavaScript

$('#msg').contents().each(function() {
    if(this.nodeType == 3) {
        var u = this.nodeValue;
        var reg = /_link_/g;
        $(this).replaceWith(u.replace(reg,'<a href="http://google.com">Google</a>'));
    }
});