I have following requirement

I have one database field whose contents are either all letters are all digits. how could i check this validaion. Please help.

i.e.
example 1

lcString = 'ABCD'

this is valid string because all characters are lettes. but if it is 'ABC1' then it is not valid because 1 inside string is not letter.

example 2

lcString = '1234'

this is valid string because all characters are digits. but if it is
'12a4' then it is not valid because 'a' inside string is not digit.

I have checked IsAlpha() and IsDigit() functions but i don't think they will be useful here. Please help me how could i check above validation and ensure that it is valid string.

Thanks for your help in advance.
Mark