PHP preg replace only allow numbers

How can I modify this existing preg_replace to only allow numbers?

function __cleanData($c) 
{
    return preg_replace("/[^A-Za-z0-9]/", "",$c);
}

Answer

I think you’re saying you want to remove all non-numeric characters. If so, D means “anything that isn’t a digit”:

preg_replace('/D/', '', $c)