JSFiddle - React, Tailwind, and code Playground

HTML

<span>Textarea / Longtest</span><br>
<input id="textarea_1" type="text" size="30" maxlength="30" value="12()1212">

<br><br><br>
    
<span>String to replace</span><br>
<input id="textarea_2" type="text" size="30" maxlength="30"><br>

<span>Replacing String</span><br>
<input id="textarea_3" type="text" size="30" maxlength="30">
    
    <input type="button" value="execute" onclick="replace()">

JavaScript

function replace(){
    var longtext = document.getElementById('textarea_1').value
    var StringToReplace = document.getElementById('textarea_2').value
    var UserReplacingString = document.getElementById('textarea_3').value;
    var oldWordRegEx = new RegExp(StringToReplace,'g');
    var result = longtext.replace(oldWordRegEx,UserReplacingString);
    
    alert(result);    
}