ROT13

rot13.cs

private static string Rot13(string inputText)
{
    return new string(inputText.Select(c =>
        {
            var lower = Char.ToLower(c);
            if (lower >= 'a' && lower <= 'z' && lower > 'm') return (char)(c - 13);
            if (lower >= 'a' && lower <= 'z') return (char)(c + 13);
            return c;
        }).ToArray());
}

rot13.fs

let rot13 (inputText:string) = 
    new string(inputText |> Seq.map(fun x ->
        let lower = System.Char.ToLower(x)
        match lower with
        | c when c >= 'a' && c <= 'z' && c > 'm' -> int x-13 |> char
        | c when c >= 'a' && c <= 'z' -> int x+13 |> char
        | _ -> x
    ) |> Seq.toArray)