JSFiddle - React, Tailwind, and code Playground

HTML

<div>==Hello==</div> with '==~~~~~=='.
<button>Replace</button>

JavaScript

$('button').click(function(){
    var original = $('div').text();
    var processed = original.replace(/(==)([^=]+)(==)/g, function(all, before, gone, after){
        return before+Array(gone.length+1).join('~')+after;
    });
    $('div').text(processed);
});