/[A]B/;     # must be 'AB'
/[A]?B/;    # 0 or 1, so 'AB' or 'B' or 'CB'
/[A]+B/;    # 1 or more, so 'AB', or 'AAAB'
/[A]*B/;    # 0 or more, so AB' or 'AAAB' or 'B' or 'CB'