Rails - Jan 2006 - cleaning a string and encoding problem

hi,
I have the following script to clean a string sent through a form:

def clean_up(string)
   chars = {
     "?" => "a",
     "?" => "e",
     "?" => "i",
     "?" => "o",
     "?" => "u",
     "?" => "n",
     " " => "-"
   }

   str = string.dup
   str.delete!(".,:(){}!|\"\\")

   for i in 0..str.length - 1 do
     char = str[i,1]
     logger.debug("log: #{char}")
     if chars.has_key?(char)
       str[i,1] = chars[char]
     end
   end

   str
end

the problem is that I have to encode this file in 
ascii mode and use the iso-8859-1 headers if I 
want it to work because otherwise the chars will 
not be recognized in the hash key set(something 
logic). so the question is, is there anyway to 
achieve this but having this file encoded in 
utf-8 like the others and having the headers in utf-8, too?

thanks in advance