Navigation php query

Hi all,

I have created a drop down nav using the following php:

// Add ID and CLASS attributes to the first
<ul> occurence in wp_page_menu
function add_menuclass($ulclass) {
return preg_replace('/</ul>
<ul>/', '<ul id="main_nav">', $ulclass, 1);
}
add_filter('wp_page_menu','add_menuclass');

This is ok because it adds main_nav to the first ul navigation. My problem is though that I also need to add ul id=sub_nav to the first child of the navigation which Im having some trouble doing, has anyone any suggestions on how I could achieve this?

Thanks in advance!

Kyle