--========================--

--== CREATE SAMPLE DATA ==--

--========================--

SELECT yourData

INTO #yourTable

FROM (VALUES('MNN.helloworld'),('GFF.goodbyeworld'),('SSW.seeyoulaterworld.oops')

)a(yourData);

--========================--

--== SOLUTION ==--

--========================--

SELECT *

FROM #yourTable

WHERE LEN(yourData)-1 = LEN(REPLACE(yourData COLLATE Latin1_General_BIN2,'.',''));