st: how to count the number of certain characters in a string variable?

[gjhxmu@sina.com]

Dear statalists,

I have a string variable X as follows:

X                Y

aabb            2

bbdd            0

cdba            1

dxbaa          2

dxaa            2

I want to generate a variable Y to stand for the number of certain characters, 
such as "aa" and replace these characters with other characters, such as "zz".


How to do it ?

Thank you !

Sincerely,