replacing.el

(defun replace-newlines-with-spaces ()
  "Replace all newlines in the current region with spaces."
  (interactive)
  (save-excursion
    (let ((start (if (region-active-p) (region-beginning) (point-min)))
          (end (if (region-active-p) (region-end) (point-max))))
      (goto-char start)
      (while (search-forward "\n" end t)
        (replace-match " " nil t)))))

(defun replace-multiple-spaces-with-single-space (beg end)
  "Replace all occurrences of multiple spaces or tabs with a single space in the selected region."
  (interactive "r")
  (save-excursion
    (goto-char beg)
    (while (re-search-forward "[ \t]+" end t)
      (replace-match " " nil nil))))

(defun replace-multiple-spaces-with-single-space (beg end)
  "Replace all occurrences of multiple spaces with a single space in the selected region."
  (interactive "r")
  (save-excursion
    ;; Get the text of the region
    (let ((text (buffer-substring-no-properties beg end)))
      ;; Replace multiple spaces with a single one
      (setq text (replace-regexp-in-string "[ \t]+" " " text))
	  ;; this is a bit of a mess:
	  ;; (setq text (replace-regexp-in-string "\\(^\\|[^\n]\\)[ \t]+\\($\\|[^\n]\\)" " " text))
      ;; Replace the region's text with the modified text
      (delete-region beg end)
      (goto-char beg)
      (insert text)
      ;; Adjust the region's boundaries to match the original text
      (setq end (+ beg (length text)))
      (set-mark end))))