Subject: 27.0.50; replace-regexp-in-string skips START first chars in return value

From: Mattias Engdegård <[email-masked]>
Date: Tue, 25 Jun 2019 14:01:49 +0200

From the doc string and the manual, the call

 (replace-regexp-in-string "a" "X" "abcab" t t nil 2)

would be expected to return

 "abcXb"

but the actual return value is

 "cXb"

This was probably not intended. The manual text is

 This function copies STRING and searches it for matches for REGEXP,
 and replaces them with REP.  It returns the modified copy.  If
 START is non-‘nil’, the search for matches starts at that index in
 STRING, so matches starting before that index are not changed.

The question is whether it is too late to fix the bug, or if it needs to be documented.

`string-match' + `replace-match' work as expected:

(let ((s "abcab"))
  (string-match "a" s 2)
  (replace-match "X" t t s))
=> "abcXb"

Bug#15107 is somewhat related.