仅当下一个字符是小写时才按大写分隔

Question

I am trying to split a string like :"ThisIsA STRING"

I want it splitted like this :"This Is A STRING"

My current regex gives me the following result :"This Is A S T R I N G"

$filname_desc = preg_replace('/([a-z0-9])?([A-Z])/','$1 $2',$filname_desc);

Is it possible to modify the regex to only split if the following character is a lowercase?

Answer 1

php> $str = "ThisIsA STRING"
php> echo preg_replace('/([a-z])([A-Z])/','$1 $2', $str)
This Is A STRING