Get first URL from custom field, download and set as featured image on post publish

I have a plugin that is adding text and a URL to a custom field when the post is published (not saved as draft). I want to get/make a function or plugin to search within the custom field and sideload the image from a URL, then add that image as the featured image.

Doing some searching I found this function from another StackExchange post that was designed to get a thumbnail from a Youtube video:

    function set_youtube_as_featured_image($post_id) {  

    // only want to do this if the post has no thumbnail
    if(!has_post_thumbnail($post_id)) { 

        // find the youtube url
        $post_array = get_post($post_id, ARRAY_A);
        $content = $post_array['post_content'];
        $youtube_id = get_youtube_id($content);

        // build the thumbnail string
        $youtube_thumb_url = 'http://img.youtube.com/vi/' . $youtube_id . '/0.jpg';

        // next, download the URL of the youtube image
        media_sideload_image($youtube_thumb_url, $post_id, 'Sample youtube image.');

        // find the most recent attachment for the given post
        $attachments = get_posts(
            array(
                'post_type' => 'attachment',
                'numberposts' => 1,
                'order' => 'ASC',
                'post_parent' => $post_id
            )
        );
        $attachment = $attachments[0];

        // and set it as the post thumbnail
        set_post_thumbnail( $post_id, $attachment->ID );

    } // end if

} // set_youtube_as_featured_image
add_action('save_post', 'set_youtube_as_featured_image');

function get_youtube_id($content) {

    // find the youtube-based URL in the post
    $urls = array();
    preg_match_all('#\bhttps?://[^\s()<>]+(?:\([\w\d]+\)|([^[:punct:]\s]|/))#', $content, $urls);
    $youtube_url = $urls[0][0];

    // next, locate the youtube video id
    $youtube_id = '';
    if(strlen(trim($youtube_url)) > 0) {
        parse_str( parse_url( $youtube_url, PHP_URL_QUERY ) );
        $youtube_id = $v;
    } // end if

    return $youtube_id; 

} // end get_youtube_id

The only thing missing here is the ability to search a custom field and pull the URL from there, although the function certainly is in the right direction with finding the URL. Can anyone give me some direction here? The text is formatted in the custom field as below. It is the only URL and has an img html tag with it.

<div class="newsread " style="margin-bottom: 20px;">
                        <div id="ncbubuhome">
                                    <p>Erdenet Mining Corp. is the Mongolian and Russian joint venture to hold copper and molybdenum mining and production of copper and molybdenum concentrates and is counted as one of the biggest manufactures in Asian region. The corporation held its annual report last Friday in Erdenet city.</p>

<p>We are delivering the numbers of Erdenet Mining Corp. in comparison with Oyu Tolgoi LLC. Although those entities have differences in their operation period and investment agreement, those two are considered as biggest projects for Mongolia.</p>                                     <p><img alt="" src="http://gstat.mn/newsn/images/ck/2015/03/31/Erdenet_Oyutolgoi_Eng-171337-1287730770.jpeg" style="height:3572px; width:825px"></p>                        </div>

I've been bashing my head against this and have tried a few things:

• Copying content of custom field to post body then using a plugin to autoset the external image (I believe things were being instantiated out of order and I couldn't get it to work)
• Putting a shortcode in the body of the article with a placeholder that would pull a custom field, but couldn't figure out how to get the placeholder to work in the text editor.

Any help is much appreciated! I've been trying this for three days now! -Robert