RangeValidation of String

Im trying to use the RangeValidation on a textbox control so that the user may only input A-Z, a-z.. therefore I am setting the Type to string, Minimum value to A and Maximum Value to z.. however its not working properly as if I just type a in the textbox, the error message fires up.

Is there anything I am missing out or should I just use a CustomValidation to check the users input?


Thanks



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