?php
$str = 'div style="float:left" id="ab">123213/div>div class="a123">213123/div>';
$search = '#[a-zA-Z0-9][^>]+?id=[^>]+?>.*?/div>#is';
preg_match_all($search,$str,$r);
echo 'pre>';
print_r($r);
echo '/pre>';
?>

Array
(
[0] => Array
(
[0] => div style="float:left" id="ab">123213/div>
)
)