PHP preg_replace: [éèêë] vs (é|è|ê|ë)

Question

I want 'é' to become 'e'.

Can someone please explain to me why $mot1 shows 'ee' when $mot2 works fine?

$mot="é";
$mot1=preg_replace("/[éèêë]/", "e", $mot);
$mot2=preg_replace("/(é|è|ê|ë)/", "e", $mot);
echo $mot1." ".$mot2;

You can test it here: http://phptester.net/

Thanks! :)

Renaud