ureech, а вот так?
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js"></script>
<script>
$(function(){
var title = "test [b]titletitletitletitle[/b] test [JS]var a = 10;[/JS] test";
title = title.replace(/\[/g, '<').replace(/\]/g, '>');
$('#thread_title').html(title);
})
</script>
</head>
<body>
<div id="thread_title"></div>
</body>
</html>