function link_words( $text ){
    //regular expression to verify if text contains tag a


    //array of words and links
    $words_to_replace = array(
    'guanciale' => '<a href="#">guanciale</a>',
    'lattuga' => '<a href="#">lattuga</a>'
    );



    foreach ($words_to_replace as $old_word => $new_word)
    {
    $reg =sprintf('/<a [^>]+>([^<]+)+[%s]+<\/a>/',$old_word);
    if(!preg_match_all($reg, $text)){
        $text = preg_replace("/$old_word/", $new_word, $text, 1);
        echo '<h1>Ho messo il link</h1>';
    }

        return $text;
    }



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