— Function: subst-char-in-region start end old-char new-char &optional noundo

This function replaces all occurrences of the character old-char with the character new-char in the region of the current buffer defined by start and end.

If noundo is non-nil, then subst-char-in-region does not record the change for undo and does not mark the buffer as modified. This was useful for controlling the old selective display feature (see Selective Display).

subst-char-in-region does not move point and returns nil.

          ---------- Buffer: foo ----------
          This is the contents of the buffer before.
          ---------- Buffer: foo ----------
          
          (subst-char-in-region 1 20 ?i ?X)
               ⇒ nil
          
          ---------- Buffer: foo ----------
          ThXs Xs the contents of the buffer before.
          ---------- Buffer: foo ----------

— Command: translate-region start end table

This function applies a translation table to the characters in the buffer between positions start and end.

The translation table table is a string or a char-table; (aref table ochar) gives the translated character corresponding to ochar. If table is a string, any characters with codes larger than the length of table are not altered by the translation.

The return value of translate-region is the number of characters that were actually changed by the translation. This does not count characters that were mapped into themselves in the translation table.