I recently ran across a situation where I need to strip a heavily nested html list such that only the top level was preserved.  I started with a regular expression solution, but found that I kept matching the wrong closing ul with an outer opening ul.

This was my alternative solution, and it seems to work well:

<?php

function stripNestedLists($str)
{
    $str2 = $str;
    $lastStr = $str2;
    
    do
    {
        // Find the first closing ul
        $cul = strpos($str2, '</ul>');
        $ul = 0;
        $lastUL = 0;
        do
        {
            // Find the next opening ul
            $lastUL = $ul;
            $ul = strpos($str2, '<ul', $ul+1);
        }
        while ($ul !== false && $ul < $cul);
    
        $lastStr = $str2;
        $str2 = substr_replace($str2, '', $lastUL, $cul-$lastUL+5);
        $str2 = trim($str2);
    }
    while (strlen($str2) > 0);
    
    return $lastStr;
}

?>

Hope this helps someone.