Replace text but keep case

I have a set of strings that I need to replace, but I need to keep the case of letters.

Both the input words and output words are of the same length.

For example, if I need to replace "abcd" with "qwer", then the following should happen:

"AbcD" translates to "QweR"
"abCd" translates to "qwEr"

and so on.

Right now I'm using JavaScript's replace, but capital letters are lost on translation.

r = new RegExp( "(" + 'asdf' + ")" , 'gi' );
"oooAsdFoooo".replace(r, "qwer");

Any help would be appreciated.