如何使用preg_replace替换重复文本？

I often have comment forms and lots of users post things such as !!!!!!!!!!! and ??????????, and I want to use preg_replace to change them to !! and ??, with two maximum.

Any idea how to do this?

$t = "aaaaaabbbbbbbbccccccccccc";
$t = preg_replace('~(.)\1{2,}~', '$1$1', $t);
echo $t; // aabbcc