add_filter( 'the_content', 'cn_nf_url_parse');

function cn_nf_url_parse( $content ) {

$regexp = "<as[^>]*href=("??)([^" >]*?)\1[^>]*>";
if(preg_match_all("/$regexp/siU", $content, $matches, PREG_SET_ORDER)) {
if( !emptyempty($matches) ) {
$srcUrl = get_option('siteurl');
for ($i=0; $i < count($matches); $i++)
{
$tag = $matches[$i][0];
$tag2 = $matches[$i][0];
$url = $matches[$i][0];
$noFollow = '';
$pattern = '/targets*=s*"s*_blanks*"/';
preg_match($pattern, $tag2, $match, PREG_OFFSET_CAPTURE);
if( count($match) < 1 )
$noFollow .= ' target="_blank" ';
$pattern = '/rels*=s*"s*[n|d]ofollows*"/';
preg_match($pattern, $tag2, $match, PREG_OFFSET_CAPTURE);
if( count($match) < 1 )
$noFollow .= ' rel="nofollow" ';

$pos = strpos($url,$srcUrl);
if ($pos === false) {
$tag = rtrim ($tag,'>');
$tag .= $noFollow.'>';
$content = str_replace($tag2,$tag,$content);
}
}
}
}
$content = str_replace(']]>', ']]>', $content);
return $content;
}

function nofollow_cat_posts($text) {
global $post;
if( in_category(1) ) { // 修改這里的分類ID
$text = stripslashes(wp_rel_nofollow($text));
}
return $text;
}
add_filter('the_content', 'nofollow_cat_posts');