On Tue, Oct 27, 2015 at 12:07 AM, Markus Triska <triska@metalevel.at> wrote:
>
>
> Steps to reproduce:
>
> 1) wget http://www.metalevel.at/ei/fault1.html
>
> 2) emacs -Q fault1.html
>
> 3) M-x delete-trailing-whitespace RET
>
> This removes several non-white-space characters, including the whole
> line containing "and use it like" (line 83 in the original file),

Apparently, the call to skip-syntax-backward is altering the match data. If you try this

--- a/lisp/simple.el

+++ b/lisp/simple.el

@@ -609,7 +609,7 @@ delete-trailing-whitespace

             (start (or start (point-min))))

         (goto-char start)

         (while (re-search-forward "\\s-$" end-marker t)

-          (skip-syntax-backward "-" (line-beginning-position))

+          (save-match-data (skip-syntax-backward "-" (line-beginning-position)))

           ;; Don't delete formfeeds, even if they are considered whitespace.

           (if (looking-at-p ".*\f")

               (goto-char (match-end 0)))

or, alternatively,

@@ -609,11 +609,12 @@ delete-trailing-whitespace

             (start (or start (point-min))))

         (goto-char start)

         (while (re-search-forward "\\s-$" end-marker t)

-          (skip-syntax-backward "-" (line-beginning-position))

-          ;; Don't delete formfeeds, even if they are considered whitespace.

-          (if (looking-at-p ".*\f")

-              (goto-char (match-end 0)))

-          (delete-region (point) (match-end 0)))

+          (let ((end (match-end 0)))

+            (skip-syntax-backward "-" (line-beginning-position))

+            ;; Don't delete formfeeds, even if they are considered whitespace.

+            (if (looking-at-p ".*\f")

+                (goto-char end))

+            (delete-region (point) end)))

         ;; Delete trailing empty lines.

         (goto-char end-marker)

         (when (and (not end)

it works.

Now, the question is, should skip-syntax-backward preserve the match data, or must delete-trailing-whitespace be coded defensively? The usual policy has been that code that needs the match data preserved should make sure itself that it is so.