How to get URL of first image in a post

We can easily grab the first image from the post, and display it. 

// Get URL of first image in a post

function get_first_image() {

         global $post;

         $first_img = ''; 
         ob_start();

         ob_end_clean();

         $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);

         $first_img = $matches [1] [0];

         if(empty($first_img)){

                  $first_image = "/images/default.jpg";

        }

        return $first_image;

}


<?php echo catch_that_image() ?>