JSFiddle - React, Tailwind, and code Playground

HTML

<div class="forum-threadview-post-text">:Psdf :P</div>
<div class="forum-threadview-post-text">:Psdf</div>
<div class="forum-threadview-post-text">asd</div>

JavaScript

$(".forum-threadview-post-text:contains(':P')").html(function (_, html) {
     return html.replace(/:P/g,"<img src='http://placehold.it/10x10' />")
});