How do you replace repetitive text using preg_replace?

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

Any idea how to do this?

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