preg_match warning

I am getting the following warning on the page where I place the shortcode (runningeventscalendar.com):-

Warning: preg_match(): Delimiter must not be alphanumeric or backslash in /home/runniled/public_html/wp-content/plugins/inline-google-spreadsheet-viewer/inline-gdocs-viewer.php on line 322

I assume it is referring this section in plugin:-

private function getDocId ($key) {
$m = array();
preg_match($this->gdoc_url_regex, $key, $m);
if (!empty($m[1])) {
$id = $m[1];
} else {
$id = sanitize_title_with_dashes($key);
}
if (‘mysql’ === $this->getDocTypeByKey($key)) {
$id = hash(‘md5’, $key);
}
return $id;

What do i need to change to fix this error? Thanks

https://ww.wp.xz.cn/plugins/inline-google-spreadsheet-viewer/