gistfile1.el

(setq correct-paren-alist 
      '(( "[" . "]" )
        ( "(" . ")" )
        ( "{" . "}" )
        ( "<<" . ">>" )))

(defun insert-correct-paren ()
  (interactive)
  (let* ((closed-parens 0)
         (correct-paren ")" )
         (op-parens (mapcar 'car correct-paren-alist))
         (cl-parens (mapcar 'cdr correct-paren-alist))
         (parens-reg (mapconcat 'regexp-quote 
                                (append op-parens cl-parens)
                                "\\|"))
         (keep-loop t))
    (save-excursion
      (save-match-data
        (while (and keep-loop
                    (re-search-backward parens-reg nil t))
          (let ((cur (buffer-substring-no-properties (match-beginning 0) 
                                                     (match-end 0))))
            (cond
             ((member cur op-parens)
              (if (zerop closed-parens)
                  (progn
                    (setq keep-loop nil)
                    (setq correct-paren (cdr (assoc cur correct-paren-alist))))
                (setq closed-parens (1- closed-parens))))
             ((member cur cl-parens) (setq closed-parens (1+ closed-parens))))))))
    (insert-string correct-paren)))