auto-nofollow.php

<?php
/**
 * @package wpsmall
*/
/**
* Plugin Name: Auto Nofollow
* Plugin URI:  https://github.com/themeasia/auto-nofollow
* Description: Auto nofollow is a really simple plugin
* Version:     1.0.1
* Author:      ThemeAsia
* Author URI:  https://themeasia.net
* License:     GPL2
* License URI: https://www.gnu.org/licenses/gpl-2.0.html
* Text Domain: autonofollow
* Domain Path: /languages 
*/

add_filter('the_content', 'auto_nofollow');
add_filter('the_excerpt', 'auto_nofollow');

function auto_nofollow($content) {
    return preg_replace_callback('/<a[^>]+/', 'auto_nofollow_callback', $content);
}
function auto_nofollow_callback($matches) {
    $link = $matches[0];
    $site_link = get_bloginfo('url');

    if (strpos($link, 'rel') === false) {
        $link = preg_replace("%(href=\S(?!$site_link))%i", 'rel="nofollow" $1', $link);
    } elseif (preg_match("%href=\S(?!$site_link)%i", $link)) {
        $link = preg_replace('/rel=\S(?!nofollow)\S*/i', 'rel="nofollow"', $link);
    }
    return $link;
}