Electrical resistors (3.5 points)
=================================

A company that makes electrical resistors uses codes to
represent its products. These product codes include letters, digits
and the following four special characters: ``'-'``, ``'_'``, ``'*'`` and
``'/'``.

The head of the production department wants to know how many
occurrences are in a product code following this pattern: *a special
character followed by a digit*. For example in the product code
``'22-6489/4_97*ABC'`` there are three of such occurrences:``'-6'``,
``'/4'`` and ``'_9'``.

Write function :py:func:`resistor` that takes a product code
(:py:class:`str`) and returns the number of described occurrences
(:py:class:`int`) that contains.

Save this function in file ``resistor.py``.

Examples:

.. literalinclude:: test-resistor.txt
      :language: python
      :lines: 3-8

.. note::

   More tests are provided in file :download:`test-resistor.txt`.