How to expand a string with PHP, using the FROM and TO numbers specified within square brackets?

Let’s say I have the string http://www.example.com/images/[1-12].jpg. I would like to expand it into:

http://www.example.com/images/1.jpg
http://www.example.com/images/2.jpg
http://www.example.com/images/3.jpg
http://www.example.com/images/4.jpg
http://www.example.com/images/5.jpg
http://www.example.com/images/6.jpg
http://www.example.com/images/7.jpg
http://www.example.com/images/8.jpg
http://www.example.com/images/9.jpg
http://www.example.com/images/10.jpg
http://www.example.com/images/11.jpg
http://www.example.com/images/12.jpg

Here is my code:

$str = "http://www.example.com/images/[1-12].jpg";
while(preg_match_all("/^([^\\[\\]]*)\\[(\\d+)\\-(\\d+)\\](.*)$/m", $str, $mat)){
$arr = array();
$num = sizeof($mat[0]);
    for($i = 0; $i < $num; $i++){
        for($j = $mat[2][$i]; $j <= $mat[3][$i]; $j++){
        $arr[] = rtrim($mat[1][$i].$j.$mat[4][$i]);
        }
    }
$str = implode(PHP_EOL, $arr);
}

It works fine even if I change $str to a more complex expression like the following:

$str = "http://www.example.com/images/[1-4]/[5-8]/[9-14].jpg";

But, unfortunately, zero-padded integers are not supported. So, if I begin with:

$str = "http://www.example.com/images/[001-004].jpg";

Expected result:

http://www.example.com/images/001.jpg
http://www.example.com/images/002.jpg
http://www.example.com/images/003.jpg
http://www.example.com/images/004.jpg

And the actual result:

http://www.example.com/images/001.jpg
http://www.example.com/images/2.jpg
http://www.example.com/images/3.jpg
http://www.example.com/images/4.jpg

How to change this behaviour so that my code produces the expected result? Also, what are the other shortcomings of my code? Should I really do it with the while loop and preg_match_all or are there faster alternatives?

UPDATE: Changing the seventh line of my code into

$arr[] = rtrim($mat[1][$i].str_pad($j, strlen($mat[2][$i]), 0, STR_PAD_LEFT).$mat[4][$i]);

seems to do the trick. Thanks a lot to sjagr for suggesting this. My question is still open because I would like to know the shortcomings of my code and faster alternatives (if any).