Replace Markup With Real Code

what I am trying to do is the following, its causing some probs though:

I have the following in a database:
[[imagediv 100.jpg This is the description text]]
This is just some dummy text. It doesn't mean anything and is for placeholding use only.

And want to change it to:
<div class="imagediv"><img src="/images/100.jpg" alt="100.jpg" width="200"
height="200" /><br />
This is the description text</div>

This is just some dummy text. It doesn't mean anything and is for placeholding use only. In the future I might want to use different codes ie [[imagediv 100.jpg This is the description text]] to reference a different DIV.