Hello dmg2016,
I think it's right.
Here at my house works well, monday in my office try the new data validation.
Your formula in italian is:

=MATR.SOMMA.PRODOTTO(LUNGHEZZA(C13)-LUNGHEZZA(SOSTITUISCI(C13;$J$13:$J$20;"")))=0

Thanks so much.
Hello everybody.
max_max