>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.

StrFilter() will handle that:

lStringIsOk=lcString==StrFilter(lcString,"0123456789")

This function is available if you

Set Library to FoxTools