public void LetterOnlyWorks()
{
Assert.AreEqual(string.Empty, MHString.LetterOnly(""), "t1");
Assert.AreEqual("ABC", MHString.LetterOnly(@"123ABC#*# -^"), "t2");
Assert.AreEqual(string.Empty, MHString.LetterOnly(@"1232&*#^#@( ~"), "t3");
}