Wordpress • Automatically adds No Follow / target='_blank" to all external links in post content / page content

AddNoFollowExternalLinks.php

<?php

class AddNoFollowExternalLinks {

    public function __construct() {

        add_filter( 'the_content', [$this, 'UrlParsing']);

    }

    public function GetLocalDomain() {
        return $_SERVER['HTTP_HOST'];
    }

    public function IsInternalLink($url) {
        
        $result = preg_match('/href(\s)*=(\s)*"[#|\/]*[a-zA-Z0-9-_\/]+"/', $url);

        if ($result) {
            return true;
        }

        $pos = strpos($url, $this->GetLocalDomain());
        if ($pos !== false) {
            return true;
        }

        return false;

    }

    public function IsLinkExists($content = '') {

        if ($content == '') {
            return null;
        }

        $regex = "<a\s[^>]*href=(\"??)([^\" >]*?)\\1[^>]*>";

        if(preg_match_all("/$regex/siU", $content, $links, PREG_SET_ORDER)) {
            return $links;
        }

        return null;

    }

    public function AddTargetBlank($url, $tag) {

        $noFollow   = '';
        $pattern    = '/target\s*=\s*"\s*_(blank|parent|self|top)\s*"/';

        if (preg_match($pattern, $url) === 0) {
            $noFollow .= ' target="_blank"';
        }

        if ($noFollow) {
            $tag = $this->UpdateCloseTag($tag, $noFollow);
        }

        return $tag;

    }

    public function AddRelNofollow($url, $tag) {

        $noFollow   = '';
        $pattern    = '/rel\s*=\s*\"[a-zA-Z0-9_\s]*\"/';

        $result     = preg_match($pattern, $url, $match);

        if ($result === 0) {

            $noFollow .= ' rel="nofollow"';

        } else {

            if (strpos($match[0], 'nofollow') === false && strpos($match[0], 'dofollow') === false) {

                $temp   = substr_replace($match[0], ' nofollow"', -1);
                $tag    = str_replace($match[0], $temp, $tag);

            }

        }

        if ($noFollow) {
            $tag = $this->UpdateCloseTag($tag, $noFollow);
        }

        return $tag;
    }

    public function UpdateCloseTag($tag, $noFollow) {

        return substr_replace($tag, $noFollow . '>', -1);

    }

    public function UrlParsing( $content ) {

        $matches = $this->IsLinkExists($content);

        if ($matches === null) {
            return $content;
        }

        // loop through each links
        for ($i=0; $i < count($matches); $i++)
        {
            $tag  = $matches[$i][0];
            $url  = $matches[$i][0];

            if(! $this->IsInternalLink($url)) {

                $tag = $this->AddTargetBlank($url, $tag);

                //exclude domain or add nofollow
                $tag = $this->AddRelNofollow($url, $tag);

                $content = str_replace($url, $tag, $content);

            }

        } // end for loop

        $content = str_replace(']]>', ']]&gt;', $content);

        return $content;

    }

}

Just include the file or load it with your autoloader and do a simple new AddNoFollowExternalLinks();