digit-char-p

(digit-char-p character &optional radix)

Returns the integer weight of character as a digit in the given radix (default 10), or nil if it is not a valid digit in that radix. For example #\7 is 7, and #\f is 15 in radix 16. Letters are accepted as digits for radices above 10, case-insensitively. A non-character signals a type-error naming the operator, and so does a radix that is not an integer from 2 to 36 (the type is (INTEGER 2 36)).