You can try this:
var myString = "EMA123_33";
var onlyLetters = new String(myString.Where(Char.IsLetter).ToArray());
please note: this version will find “e” just like “E” – if you need only upper-case letters then do something like this:
var myString = "EMA123_33";
var onlyLetters = new String(myString.Where(c => Char.IsLetter(c) && Char.IsUpper(c)).ToArray());