How to match "C", "C++" or "C#"?

I have the following test cases:

CPP_CSHARP_REGEX_TEST_CASES = [
    ("C++", True),
    ("C#", True),
    ("C+", False),
    ("C##", False),
    ("C", True),
]

Which regular expression could I use to pass pytest?

Assuming you expect these matches to be the entire input string, then use:

^C(?:\+\+|#)?$

If you want to catch these matches perhaps as part of a larger string, with the matches surrounded by whitespace, then use:

\bC(?:\+\+|#)?(?!\S)