at https://docs.python.org/3/howto/regex.html#regex-howto
and
https://docs.python.org/3.8/howto/regex.html#regex-howto
https://docs.python.org/3.7/howto/regex.html#regex-howto
https://docs.python.org/3.6/howto/regex.html#regex-howto
https://docs.python.org/3.5/howto/regex.html#regex-howto
https://docs.python.org/3.4/howto/regex.html#regex-howto
https://docs.python.org/2.7/howto/regex.html#regex-howto

The following paragraph seems to have a small issue:
"
You can match the characters not listed within the class by complementing the set. This is indicated by including a '^' as the first character of the class; '^' outside a character class will simply match the '^' character. For example, [^5] will match any character except '5'.
"
^ does not simply match ^ outside a character class since is a special character that represents the start of the string. 

I think the paragraph should read: 
You can match the characters not listed within the class by complementing the set. This is indicated by including a '^' as the first character of the class; '^' will act differently outside a character class as explained later. For example, [^5] will match any character except '5'.