仅替换多个新行

I am trying to do something kind of unusual. I want to replace multiple new lines with two new lines. There are plenty of questions like that but not exactly, see:

Hello World !

Should become

Hello World !

This however

Hello World !

Should stay the same. The problem with my regex is that it replaces single new lines with two.

preg_replace('"(? )+"', " ", $somevar)

something like this?

preg_replace('"(?
){2,}"', "

", $somevar)