This is what you should get:

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
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That's exactly what you get. What did you expect to be different?