Only apply code to divs within a specific div?

Only apply code to divs within a specific div?

I have a page where the markup looks like this:

<div id="maindiv">
<div id="post1"> content </div>
</div>

<div id="subdiv">
<div id="post3"> content </div>
<div id="post5"> content</div>
</div>

I'm trying to pull *only* the ids of the posts within the subdiv container. The code pops up an alert box for post1, post3, and post5, but I only want it to return post3 and post5...

$j('div[id^="post-"]').each(function() {
                alert(this.id);
        });


/* Tried this */
#j("#subdiv").each(function() {
        $j('div[id^="post-"]').each(function() {
                alert(this.id);
        });
});

But, it still return all three ids...[/code]