6. Regex quantifier 

/X?/;             # 0 or 1 X
/X*/;             # 0 or more Xs
/X+/;             # 1 or more Xs

/X{3}/;           # Exactly 3 Xs
/X{4,}/;          # At least 4 Xs
/X{2,5}/;         # At least 2 but no more than 5 Xs

• Reference: perlre/Quantifiers