Detect gist shortcode in post content

gistfile1.php

<?php
/** 
 * Detect gist shortcode in post content
 *
 * This assumes you are using a shortcode in the format [gist id="xxxxx'].
 * I use this code in a custom metabox that is registered for my code snippets
 * custom post type. This way after I save the snippet (or when editting an
 * existing snippet) it will show the snippet below the post editor in a
 * metabox. 
 *
 * @author Jared Atchison
 * @link http://jaredatchison.com/code/
 */

// Drop this inside your custom metabox

global $post;

if ( preg_match( '/\[gist[^\]]*\]/siU', $post->post_content, $matches ) ) {
	if ( $matches[1] ) {
		echo '<script src="https://gist.github.com/' . $matches[1] . '.js"> </script>';		
	}
} else {
	echo 'No gist detected.';
}