Add an option to preserve ANSI sequences

* lisp/ansi-color.el Add an option to preserve the ANSI sequences
* test/lisp/ansi-color-tests.el: Add tests (bug#44589).

lisp/ansi-color.el

@@ -363,7 +363,7 @@ it will override BEGIN, the start of the region.  Set
 	  (setq ansi-color-context-region (list nil (match-beginning 0)))
 	(setq ansi-color-context-region nil)))))
 
-(defun ansi-color-apply-on-region (begin end)
+(defun ansi-color-apply-on-region (begin end &optional preserve-sequences)
   "Translates SGR control sequences into overlays or extents.
 Delete all other control sequences without processing them.
 
@@ -380,18 +380,28 @@ ansi codes.  This information will be used for the next call to
 `ansi-color-apply-on-region'.  Specifically, it will override
 BEGIN, the start of the region and set the face with which to
 start.  Set `ansi-color-context-region' to nil if you don't want
-this."
+this.
+
+If PRESERVE-SEQUENCES is t, the sequences are hidden instead of
+being deleted."
   (let ((codes (car ansi-color-context-region))
-	(start-marker (or (cadr ansi-color-context-region)
-			  (copy-marker begin)))
-	(end-marker (copy-marker end)))
+        (start-marker (or (cadr ansi-color-context-region)
+                          (copy-marker begin)))
+        (end-marker (copy-marker end)))
     (save-excursion
       (goto-char start-marker)
       ;; Find the next escape sequence.
       (while (re-search-forward ansi-color-control-seq-regexp end-marker t)
-        ;; Remove escape sequence.
-        (let ((esc-seq (delete-and-extract-region
+        ;; Extract escape sequence.
+        (let ((esc-seq (buffer-substring
                         (match-beginning 0) (point))))
+          (if preserve-sequences
+              ;; Make the escape sequence transparent.
+              (overlay-put (make-overlay (match-beginning 0) (point))
+                           'invisible t)
+            ;; Otherwise, strip.
+            (delete-region (match-beginning 0) (point)))
+
           ;; Colorize the old block from start to end using old face.
           (funcall ansi-color-apply-face-function
                    (prog1 (marker-position start-marker)

test/lisp/ansi-color-tests.el

@@ -0,0 +1,49 @@
+;;; ansi-color-tests.el --- Test suite for ansi-color  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2020 Free Software Foundation, Inc.
+
+;; Author: Pablo Barbáchano <pablob@amazon.com>
+;; Keywords: ansi
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'ansi-color)
+
+(defvar test-strings '(("\e[33mHello World\e[0m" . "Hello World")
+                       ("\e[1m\e[3m\e[5mbold italics blink\e[0m" . "bold italics blink")))
+
+(ert-deftest ansi-color-apply-on-region-test ()
+    (dolist (pair test-strings)
+      (with-temp-buffer
+        (insert (car pair))
+        (ansi-color-apply-on-region (point-min) (point-max))
+        (should (equal (buffer-string) (cdr pair)))
+        (should (not (equal (overlays-at (point-min)) nil))))))
+
+(ert-deftest ansi-color-apply-on-region-preserving-test ()
+    (dolist (pair test-strings)
+      (with-temp-buffer
+        (insert (car pair))
+        (ansi-color-apply-on-region (point-min) (point-max) t)
+        (should (equal (buffer-string) (car pair))))))
+
+(provide 'ansi-color-tests)
+
+;;; ansi-color-tests.el ends here