assign different classes to php array
===

I am working on some images and want to assign different classes to them in batch.

here is my simple code 

    <?php
    $image_dir = 'images';
    $showimage = scandir($image_dir, 0);
    unset($showimage [0],$showimage [1]);
    print_r ($showimage );
    ?>

the result is 

> Array ( [2] => exmapleiamge1.png [3] => exmapleiamge1_1.png [4] =>
> exmapleiamge1_2.png [5] => exmapleiamge2_1.png [6] =>
> exmapleiamge2_2.png [7] => exmapleiamge2_3.png [8] =>
> exmapleiamge3_1.png [9] => exmapleiamge3_2.png )

What I want to achieve is that, assign div class=no1 to all images from exmapleiamge1_1 to exmapleiamge1_2, and assign class=no2 images from exmapleiamge2_1 to exmapleiamge2_3, and also for class no3.... etc;

I think the key is to filter the numbers, is it possible someone could give me a hand as I am a php beginner and keen to learn how to do that.  

Many thanks in advance