$buf = preg_replace('#viewthread\.php\?forum_id=([0-9]*?)(&|&)thread_id=([0-9]*?)(&|&)pid=([0-9]*?)\#post_([0-9]*?)(\'|")#si', 'forumas\1-\3-\5_postas\6\7', $buf);
RewriteRule ^forumas-([0-9]+)-postas-([0-9]+)-psl([0-9]+)$ forumas/viewthread.php?rowstart=$3&forum_id=$1&thread_id=$2 [L]