add_filter( 'the_content', 'cn_nf_url_parse');
 
function cn_nf_url_parse( $content ) {
 
	$regexp = "<a\s[^>]*href=(\"??)([^\" >]*?)\\1[^>]*>";
	if(preg_match_all("/$regexp/siU", $content, $matches, PREG_SET_ORDER)) {
		if( !empty($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 = '';
 
				$pattern = '/target\s*=\s*"\s*_blank\s*"/';
				preg_match($pattern, $tag2, $match, PREG_OFFSET_CAPTURE);
				if( count($match) < 1 )
					$noFollow .= ' target="_blank" ';
 
				$pattern = '/rel\s*=\s*"\s*[n|d]ofollow\s*"/';
				preg_match($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;
 
}