code: def check_mac(original_mac):
      '''
      Checks the format of a MAC address and returns it without double-colons and in capital letters, if it is correct. Otherwise it returns None.
      * it accepts the format of the double colons and a single hex string
      '''
      mac = (original_mac.upper()).strip()
      parts = mac.split(':')
      if len(parts) == 6:
          # let's think that it is a : separated mac
          for p in parts:
              if len(p) != 2:
                  return None
          mac = ''.join(parts)
      elif len(parts) > 1:
          return None
      for c in mac:
          if c not in '0123456789ABCDEF':
              return None
      return maf check_mac(original_mac):
      '''
      Checks the format of a MAC address and returns it without double-colons and in capital letters, if it is correct. Otherwise it returns None.
      * it accepts the format of the double colons and a single hex string
      '''
      mac = (original_mac.upper()).strip()
      parts = mac.split(':')
      if len(parts) == 6:
          # let's think that it is a : separated mac
          for p in parts:
              if len(p) != 2:
                  return None
          mac = ''.join(parts)
      elif len(parts) > 1:
          return None
      for c in mac:
          if c not in '0123456789ABCDEF':
              return None
      return mac
docs: Checks the format of a MAC address and returns it without double-colons and in capital letters, if it is correct. Otherwise it returns None.
      * it accepts the format of the double colons and a single hex string