replace-regexp-in-string/replace-regexp/replace-string

戻り値をごにょごにょしたいときは、replace-regexp-in-stringを使うと良い

(replace-regexp-in-string "test abc" "" "test abcdef")
"def"
(replace-regexp "test abc" "" "test abcdef")
nil
(replace-string "test abc" "" "test abcdef")
nil

文字列 - 逆引きEmacs Lisp - アットウィキ