words:   aa as es is os sh si so
string:  aa as es is os hs is os
^.?a?$:  y  n  n  n  n  n  n  n    aa
^a?.?$:  y  y  n  n  n  n  n  n    aa as
^s?.?$:  n  n  n  n  n  n  n  n    
^.?s?$:  n  y  y  y  y  y  y  y    as es is os sh si so