Regrex substitution!!!

[Fernando]

I have a credit card number that I want to change to email a reciept to the customer. 
This is that I want:

I have this number: e.j. 1111 8578 596 8552
I want to convert all the number to "x" like that xxxx xxx xxxx

when I use this:

$number = "1111 8578 596 8552";
$number =~ s/\d+/x/g;

Perl give only one "x" in the result.

If any body can help me please!!!