To add the "nofollow" attribute to external links in WordPress blog articles, you can add the following code to the functions.php file of your theme. This code will only add the "nofollow" attribute to external links and will not change the attributes of internal links.

// Automatically add nofollow attribute to external links in the content
add_filter('the_content', 'web589_the_content_nofollow', 999);
function web589_the_content_nofollow($content){
    preg_match_all('/href="(.*?)"/', $content, $matches);
    if($matches){
        foreach($matches[1] as $val){
            if( strpos($val, home_url()) === false ) {
                $content = str_replace("href=\"$val\"", "href=\"$val\" rel=\"external nofollow\"", $content);
            }
        }
    }
    return $content;
}

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