function autoblank($text) {

    $search = array(
        '<a',
        'target="_blank" href="URL',
        'target="_blank" href="/',
        'target="_blank" href="#',
        ' target = "_blank">'
    );
    $replace = array(
        '<a target="_blank"',
        'href="URL',
        'href="/',
        'href="#',
        '>'
    );
    $string = str_ireplace($search, $replace, $string);

    return $string;
}
add_filter('the_content', 'autoblank');
add_filter('comment_text', 'autoblank');