Multiple replacement with StringRegExpReplace

Hello,

Could you please tell me what's wrong in the following instruction :

$Output = StringRegExpReplace($Input, "[àâäçéèêëîïñôöùûüÀÂÄÇÉÈÊËÎÏÑÔÖÙÛÜ]", "[aaaceeeeiinoouuuAAACEEEEIINOOUUU]")

I thought it would replace each character one by one, like the following sed command :
sed "y/[àâäçéèêëîïñôöùûüÀÂÄÇÉÈÊËÎÏÑÔÖÙÛÜ]/[aaaceeeeiinoouuuAAACEEEEIINOOUUU]/" Input.txt > Output.txt

But apparently, the destination regexp is taken as a whole.

For example, I get :
Déjà -> D[aaaceeeeiinoouuuAAACEEEEIINOOUUU]j[aaaceeeeiinoouuuAAACEEEEIINOOUUU]
instead of :
Déjà -> Deja

Is there a simple command for multiple regexp vs. regexp replacement ?
If no, I'll do some kind of loop to replace each characters one by one.

Cheers !