class Mail::Multibyte::Chars

def []=(*args)

# => "Möler"
s
s.mb_chars[1, 2] = "ö" # Replace 2 characters at character offset 1
s = "Müller"

# => "Müeler"
s
s.mb_chars[2] = "e" # Replace character with offset 2
s = "Müller"

Example:

Like String#[]=, except instead of byte offsets you specify character offsets. 
def []=(*args)
  replace_by = args.pop
  # Indexed replace with regular expressions already works
  if args.first.is_a?(Regexp)
    @wrapped_string[*args] = replace_by
  else
    result = Unicode.u_unpack(@wrapped_string)
    if args[0].is_a?(Integer)
      raise IndexError, "index #{args[0]} out of string" if args[0] >= result.length
      min = args[0]
      max = args[1].nil? ? min : (min + args[1] - 1)
      range = Range.new(min, max)
      replace_by = [replace_by].pack('U') if replace_by.is_a?(Integer)
    elsif args.first.is_a?(Range)
      raise RangeError, "#{args[0]} out of range" if args[0].min >= result.length
      range = args[0]
    else
      needle = args[0].to_s
      min = index(needle)
      max = min + Unicode.u_unpack(needle).length - 1
      range = Range.new(min, max)
    end
    result[range] = Unicode.u_unpack(replace_by)
    @wrapped_string.replace(result.pack('U*'))
  end
end