urlcounter.php

<?php
/*
Plugin Name: URL counter
Plugin URI:  http://URI_Of_Page_Describing_Plugin_and_Updates
Description: This describes my plugin in a short sentence
Version:     1.5
Author:      Tom Woodward
Author URI:  http://bionicteaching.com
License:     GPL2
License URI: https://www.gnu.org/licenses/gpl-2.0.html
Domain Path: /languages
Text Domain: my-toolset

*/defined( 'ABSPATH' ) or die( 'No script kiddies please!' );

//the actions that trigger the function
add_action('save_post', 'countURLs');
add_action('draft_to_publish', 'countURLs');
add_action('pending_to_publish', 'countURLs');

function countURLs ($post_id){
		$new_post = get_post($post_id);
		//gets the post text
		$content = $new_post->post_content;
		//the regex
		$pattern = '"\"https?:\/\/([^\"]*)\""';
		//apply the regex to the post put the matches in the $matches array
		preg_match_all($pattern, $content, $matches, PREG_SET_ORDER);
		//count the number of URLs in the array
		$count = (count($matches));
		//write the count of URLs to the URL_count custom field
		update_post_meta($new_post->ID, 'URL_count', $count);
}