how to get alt of image

l want to loop the post,if there is a feature image(or called thumbnail images) in post,loop the thumbnail,if not,loop the first images in content;here is my code added in functions.php

function don_the_thumbnail() {
    global $post;
    $thumbnail = get_post( get_post_thumbnail_id() );
    if ( has_post_thumbnail() ) { //judge whether there is a thumbnail in the post
        echo '<a href="'.get_the_permalink().'"><img class="img-responsive img-blog" src="'.get_the_post_thumbnail_url().'" alt="'.get_post_meta( $thumbnail->ID, '_wp_attachment_image_alt', true ).'" /></a>';
    } else { //if not,get find whether there is an image in post
        $content = $post->post_content;
        preg_match_all('/<img.*?(?: |\\t|\\r|\\n)?src=[\'"]?(.+?)[\'"]?(?:(?: |\\t|\\r|\\n)+.*?)?>/sim', $content, $strResult, PREG_PATTERN_ORDER);
        $n = count($strResult[1]);
        if($n > 0){ // if there is an image in post,get the first image
            echo '<a href="'.get_the_permalink().'"><img class="img-responsive img-blog" src="'.$strResult[1][0].'" alt="" /></a>';
        }else { // if there is no image in post,nothing get
            return;
        }
    }
}

the problem is that how do l get the alt text of the first image in the post?need a hand,thank you all.