Create User Function W/Case

I keep getting an error message "incorrect syntax near keyword case"
when trying to run this:

USE DEDUPE
GO

CREATE FUNCTION fnCleanString
(
@mString varchar (255)
)
RETURNS varchar(255)
AS
BEGIN
DECLARE
@mChar char(1),
@msTemp varchar(255),
@miLen int,
@i int,
@iAsc int
BEGIN
set @mChar = ''
set @msTemp = ''
set @miLen = Len(@mString)
set @i = 1
while @i <= @miLen
begin
set @mChar = substring(@mString,@i,1)
set @iAsc = Ascii(@mChar)
case
when @iAsc >= 87 And iAsc <= 122 Then set @mChar = @mChar
when iAsc >= 65 And iAsc <= 90 Then set @mChar = @mChar
when iAsc >= 49 And iAsc <= 57 Then set @mChar = @mChar
else @mChar = ""
end
set @msTemp = @msTemp & @mChar
set @i = @i + 1
end
END
RETURN @msTemp
END

Can anybody point out what I'm doing wrong?
Thanks.
Randy