A couple more.
; convert user input string to a list
; remove any uppercase letters
; put the string back together
(setq s-lst (vl-string->list (getstring T "\nEnter a string: ")))
(vl-list->string (vl-remove-if '(lambda (i) (and (< 65 i)(> 90 i))) s-lst))
; same as above, but removes the lowercase letters
(setq s-lst (vl-string->list (getstring T "\nEnter a string: ")))
(vl-list->string (vl-remove-if-not '(lambda (i) (and (< 64 i)(> 91 i))) s-lst))