Check input for "AS" or "as", as well as once the numbers have been extr...

...acted no other letters exist

Sanitize the input, stripping any "AS" or "as" off the front of the string

Ensure that once the leading "AS" or "as" has been removed, no letters still exist

Invalid input includes a123, 123a, 123AS, 123anything

Valid input includes: 25795, as25795,  AS25795