preg_match_all

I am using the code below to search for the first image in a post:

preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
	if ( isset( $matches ) ) {
	$url = $matches[1][0];
	}

I really want to search for the first image in a post that has an underscore “_” in the url, can this be done?