code critique

For educational purposes (resp. fun) I wrote some code for the cgi
GET method. It works like this:

(get-parameter 'username) => "John"

Since I'm learning Lisp on my own, I would highly appreciate any tips
concerning my code. What could be done better, what is already ok?

Regards,

dhl


(defun convert-hex-encoded-chars (string)
"Converts %HEX encoded chars found in string."
(if (cl-ppcre:scan "%.." string)
(cl-ppcre:register-groups-bind
(before match after) ("([^%]*)(%..)(.*)" string)
(concatenate 'string before
(string (code-char
(parse-integer (subseq match 1) :radix 16)))
(convert-hex-encoded-chars after)))
string))

(defun split-query-string (query-string)
"Splits a query-string and returns a list of variable=value assignments."
(cl-ppcre:split "&" (substitute #\Space #\+
(convert-hex-encoded-chars query-string))))

(defun get-parameters (&optional
(query-string
(osicat:environment-variable "QUERY_STRING")))
"Returns an alist of (variable . value) pairs for a cgi query-string, which
may be given as an optional parameter. Otherwise, the environment-variable
QUERY_STRING is used."
(loop for variable in (split-query-string query-string)
collect (cl-ppcre:register-groups-bind
(key value) ("(.*)=(.*)" variable)
(cons (intern (string-upcase key))
value))))

(defparameter *parameters* (get-parameters)
"Alist of QUERY_STRING variables.")

(defun get-parameter (key)
"Returns the value of a QUERY_STRING variable named key."
(cdr (assoc key *parameters*)))

.

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