String successor like ruby - I guess

string-successor.lisp

(defun char-successor(c)
  "values are succesor and carry, same case for letters"
  (cond
    ((char= c #\9) (values #\0 t))
    ((char= c #\Z) (values #\A t))
    ((char= c #\z) (values #\a t))
    ((alphanumericp c) (values (code-char (1+ (char-code c))) nil))
    (t (values c t))))

(defun string-successor(s)
  (let ((i (1- (length s)))
        (s1 (copy-seq s))
        (p1 (position-if #'alphanumericp s)))
    (when (null p1)
      (setf (char s1 i) (code-char (1+ (char-code (char s1 i)))))
      (return-from string-successor s1))      
    (loop (multiple-value-bind (succ carry) (char-successor (char s i))
            (setf (char s1 i) succ)
            (if (not carry)
                (return-from string-successor s1)
                (if (= i p1)
                    (return-from string-successor
                      (concatenate 'string
                                   (subseq s1 0 i)
                                   (string (if (char= succ #\0) #\1 succ))
                                   (subseq s1 (1+ i))))
                    (decf i)))))))

Fixed https://www.reddit.com/r/lisp/comments/40p00f/a_little_of_ruby_in_lisp/cywo54i