code: def _normalize_helper(number, replacements, remove_non_matches):
      
      normalized_number = []
      for char in number:
          new_digit = replacements.get(char.upper(), None)
          if new_digit is not None:
              normalized_number.append(new_digit)
          elif not remove_non_matches:
              normalized_number.append(char)
          # If neither of the above are true, we remove this character
      return U_EMPTY_STRING.join(normalized_number)
docs: Normalizes a string of characters representing a phone number by
      replacing all characters found in the accompanying map with the values
      therein, and stripping all other characters if remove_non_matches is true.
  
      Arguments:
      number -- a string representing a phone number
      replacements -- a mapping of characters to what they should be replaced
                by in the normalized version of the phone number
      remove_non_matches -- indicates whether characters that are not able to be
                replaced should be stripped from the number. If this is False,
                they will be left unchanged in the number.
  
      Returns the normalized string version of the phone number.