RegExp Problem using Substitutions.

[Deviloper]

Hi there!

I have a string "aaaabbbababbaaassass". I want to get a string without any 
double a 'aa' or and without the 'b's.

but if I do:

my $s = "aaaabbbababbaaassass";
$s=~ s/aa|b//g;

as a result I will get a string "aaassass".

(I understand WHY I get this result. 
But I don´t know how to avoid this. (Using only one substition.))

Thx,
B.