Preg_match from a custom field

Hi everybody!
What this plugin does is find out a Youtube video embed in the body of the post, extract the video id and automatically create a custom field with the thumbnail address (ex. https://img.youtube.com/vi/AMXkiX9Hr_o/default.jpg).
My problem is that in my video-blog I embed video by using a custom field (with key “embed”), and not adding it normally to the post.
How can I use the function “preg_match” with the custom field “embed”?
I hope to have been clear… sorry for my english and thanks for your help!

<?php
/*
Plugin Name: YouTubeThumb2CustomField
Plugin URI: https://herrpfleger.de/wp-plugin
Description: Inserts URL to YouTube-thumbnail into Custom Field when you embed YouTube video into your post.
Author: Herr Pfleger
Version: 0.8
Author URI: https://herrpfleger.de
*/

add_action(‘save_post’,’find_ytid’, 10, 2);

function find_ytid($postID, $post) {
if($parent_id = wp_is_post_revision($postID))
{
$postID = $parent_id;
}
$content = $_POST[‘content’];
if (preg_match(‘/http:\/\/www.youtube\.com\/v\/([a-zA-Z0-9\-\_]{11})/’, $content, $yturl) !=”) {
$ytid = substr($yturl[0], 25, 31);
$custom = ‘https://img.youtube.com/vi/&#8217;.$ytid.’/hqdefault.jpg’;
update_custom_meta($postID, $custom ,’ytthumb_url’);
}
elseif (preg_match(‘/http(v|vh|vhd):\/\/([a-zA-Z0-9\-\_]+\.|)youtube\.com\/watch(\?v\=|\/v\/)([a-zA-Z0-9\-\_]{11})([^<\s]*)/’, $content, $yturl) !=”) {
$custom = ‘https://img.youtube.com/vi/&#8217;.$yturl[4].’/hqdefault.jpg’;
update_custom_meta($postID, $custom ,’ytthumb_url’);
}
}

function update_custom_meta($postID, $newvalue, $field_name) {
// To create new meta
if(!get_post_meta($postID, $field_name)){
add_post_meta($postID, $field_name, $newvalue);
}else{
// or to update existing meta
update_post_meta($postID, $field_name, $newvalue);
}
}
?>