elisp find/replace efficiency question

[Xah Lee]

suppose i want write a command that do a find/replace on a region

which is more efficient?

(defun space2underscore-region (p1 p2)
  "Replace space by underscore in region."
  (interactive "r")
  (save-restriction
    (narrow-to-region p1 p2)
    (goto-char (point-min))
    (while (search-forward " " nil t) (replace-match "_")) ) )

vs

...
(setq meat (replace-regexp-in-string " " "_"
 (buffer-substring-no-properties p1 p2)) )
(delete-region p1 p2)
(insert meat)

vs

(replace-string " " "_" nil p1 p2)

it appears to me, the first is most efficient but is most cumbersome.
The last is most lean, but least efficient.


(sorry if this is a duplicate post)

 Xah ∑ xahlee.org ☄