Nofollow all external links inside posts

functions.php

<?php

// nofollow all external links in posts and pages
add_filter('the_content', 'vc_nofollow');
add_filter('the_excerpt', 'vc_nofollow');

function vc_nofollow($content) {
      return preg_replace_callback('/<a[^>]+/', 'vc_nofollow_callback', $content);
}
function vc_nofollow_callback($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;
}