A quick-n-dirty solution to https://techdevguide.withgoogle.com/paths/advanced/compress-decompression/#code-challenge

google-challenge.lisp

(ql:quickload :cl-ppcre)

(defun decompress (in)                                                                                                                                                                                               
  (multiple-value-bind (string matchp)                                                                                                                                                                               
      (cl-ppcre:regex-replace "([0-9]+)\\[([^][]*)\\]"                                                                                                                                                                
                              in                                                                                                                                                                                     
                              (lambda (match reg1 reg2)                                                                                                                                                              
                                (declare (ignorable match))                                                                                                                                                          
                                (format nil "~v@{~A~:*~}" (parse-integer reg1) reg2))                                                                                                                                
                              :simple-calls t                                                                                                                                                                        
                              :preserve-case t)                                                                                                                                                                      
    (if matchp                                                                                                                                                                                                       
        (decompress string)                                                                                                                                                                                          
        string)))

;; (decompress "3[abc]4[ab]c") => "abcabcabcababababc"
;; (decompress "2[3[a]b]")     => "aaabaaab"