var abc = new List<string> { "abc", "123", "abd12" };
var alphaXorNumerical = abc.Where(str => str.All(Char.IsDigit) ||
str.All(Char.IsLetter));
var others = abc.Except(alphaXorNumerical);
If you also want to check for whitespace, use this instead:
var alphaXorNumerical = abc
.Where(str => str.All(Char.IsDigit) ||
str.All(ch => Char.IsLetter(ch) || Char.IsWhiteSpace(ch)));
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solved extracting specific format elements from the list in c#