Thrive Content Editor - Add nofollow to external links

function.php

<?php

function add_nofollow( $content ) {
    $re = '/<a(.*)>/iU';
    preg_match_all( $re, $content, $matches );
    $domain = get_bloginfo( 'url' );
    foreach ( $matches[0] as $link ) {
        if ( strpos( $link, $domain ) === false ) {
            if ( strpos( $link, 'nofollow' ) === false ) {
                $new_link = str_replace( '>', ' rel="nofollow">', $link );
                $content  = str_replace( $link, $new_link, $content );
            }
        }
    }

    return $content;
}

add_filter( 'tcb_clean_frontend_content', 'add_nofollow' );