Preg Replace a curly apostrophe?

I’m trying to use preg replace to replace a single curly postrophe (amongst other characters but I have excluded them from my example).

preg_replace('/[‘]/', 'x', '‘');

Unfortunately the above outputs xxx, I would expect to only see a single x. Where am I going wrong?

>Solution :

Make it unicode capable by adding the u flag.

echo preg_replace("/[‘]/u", 'x', '‘');

prints

x