The regex says "Match any number of 'B's, including none, from the beginning of the line to the end." So, an empty line would match; so would a line containing nothing but a single 'B'; ditto for a string consisting of nothing but 'B's of any length - but that's the entire range of what that regex will match. 'C' isn't one of the above choices, so it doesn't match.

-- 
Education is not the filling of a pail, but the lighting of a fire.
 -- W. B. Yeats