369-4 A question regarding the boundaries

The task was to write a regular expression that includes ‘C’ or ‘c’ and excludes ‘C++’ or ‘C.’

My Code:

pattern = r"\b[Cc][^+.]\b"

Solution Code:

pattern = r"\b[Cc]\b[^+.]"

I don’t understand why the boundaries need to exclude [^+.]

+ is not a word.