JavaScript Make new BB code

[System32]

For example, I want to have on my forum, inside post, text like this:
[mark] SOME TEXT [/mark]

Now, what I need is a jQuery script that will take complete block, do some CSS on "SOME TEXT" and remove [mark][/mark]. But I want that to be applied throughout that post. For example, class where whole content is placed is .post-content
Any ideas how this can be done?

 

[Johna]

You could do something like this:

<textarea id="bbcode"></textarea>
<button id="submit">Submit</button>
<p id="result"></p>

$("#submit").click(function() {
    $("#result").html($("#bbcode").val().replace(/\[mark\]/gi, "<span style='color:red;'>"));
    $("#result").html($("#result").html().replace(/\[\/mark\]/gi, "</span>"));
})

Here's a jsfiddle:

https://jsfiddle.net/pfxzt65n/1/