APEX

Hi,
I'm trying to use a regular expression to validate password.
I create a validation in my item called P502_PASSWORD. I choose regular expression type. At the Validation Expression 2, I put this expression:

^(?=.*\d)(?=.*[a-zA-Z])(?!.*[\W_\x7B-\xFF]).{6,15}$

So, I tried to enter a valid "string" at this item. For example: abc123

But it doesn't work and I don't know why. I have another item with a e-mail validation with the expression below, and works:

^[A-Za-z0-9._%-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$

So, i really don't know what is happening.
Can someone help me? :)

Thanks,
Vivi