“aaaa1234bbbb” => “aaaabbbb”

# sh
% echo "aaaa1234bbbb" | sed -e 's/^\([a-z]*\)\([0-9]*\)\([a-z]*\)$/\1\3/'
aaaabbbb

;; elisp(文字列の走査)
(let ((str "aaaa1234bbbb"))
  (string-match "\\([a-z]+\\)\\([0-9]+\\)\\([a-z]+\\)" str)
  (concat
   (match-string-no-properties 1 str)
   (match-string-no-properties 3 str)))
=> "aaaabbbb"

;; elisp(バッファの走査)        <= ちょっと大げさ
(with-temp-buffer
  (let ((str "aaaa1234bbbb"))
    (insert str)
    (goto-char (point-min))
    (if (re-search-forward "\\([a-z]+\\)\\([0-9]+\\)\\([a-z]+\\)" (point-max) t)
       (concat
        (match-string-no-properties 1)
        (match-string-no-properties 3)))))
=> "aaaabbbb"