ДоХтор, вот так сделаль не получилас
$post = '@udesign test';
preg_match('~\s@([\w]+)~iu', $post, $match);
$udes_name = print_r($match[1]);
$ud_st2 = mysql_query("SELECT * FROM `users` where `name` = '". $udes_name ."'");
$ud_st = mysql_fetch_assoc($ud_st2);
echo '<a href="http://' . $_SERVER['HTTP_HOST']. '/users/profile.php?user=' . $ud_st['id'] . '">'; print_r($match[0]); echo '</a>';
echo $post;