negate the set of characters in python regular expression: [^\/ab]

https://github.com/tartley/python-regex-cheatsheet/blob/master/cheatsheet.rst


After '[', enclose a set, the only special chars are:

]   End the set, if not the 1st char
-   A range, eg. a-c matches a, b or c
^   Negate the set only if it is the 1st char