GNU bug report logs - #21253
sed escape bug

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Jacob Young <amazingjacob <at> gmail.com>
To: bug-sed <at> gnu.org
Subject: sed escape bug
Date: Thu, 13 Aug 2015 08:24:33 -0400

[Message part 1 (text/plain, inline)]

The escape '\c\' fails at the end of a regex.

~$ echo a | sed -e 's/a/\c\/' | hexdump -c
sed: -e expression #1, char 8: unterminated `s' command
~$ echo a | sed -e 's/a/\c\b/' | hexdump -c
0000000 034   b  \n
0000003
~$ sed --version
sed (GNU sed) 4.2.2
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html
>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Jay Fenlason, Tom Lord, Ken Pizzini,
and Paolo Bonzini.
GNU sed home page: <http://www.gnu.org/software/sed/>.
General help using GNU software: <http://www.gnu.org/gethelp/>.
E-mail bug reports to: <bug-sed <at> gnu.org>.
Be sure to include the word ``sed'' somewhere in the ``Subject:'' field.

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Message #10 received at 21253-done <at> debbugs.gnu.org (full text, mbox):

From: Assaf Gordon <assafgordon <at> gmail.com>
To: 21253-done <at> debbugs.gnu.org
Subject: RE: sed escape bug
Date: Thu, 16 Feb 2017 22:52:32 +0000

Hello,

Sorry for the delayed response.


Jacob Young <amazingjacob <at> gmail.com> wrote:
> The escape '\c\' fails at the end of a regex.
>
> ~$ echo a | sed -e 's/a/\c\/' | hexdump -c
> sed: -e expression #1, char 8: unterminated `s' command
> ~$ echo a | sed -e 's/a/\c\b/' | hexdump -c
> 0000000 034   b  \n
> 0000003
> ~$ sed --version
> sed (GNU sed) 4.2.2

This is an interesting case:
It's a bug in older sed-4.2.2, but the fix is perhaps not your expected 
behaviour of allowing '/' after '\c\'.

A single backslash after '\c' was ambigious in sed-4.2.2:
Should it be parsed as the option to '\c' (e.g. '\c\' is like '\cA'),
or should it be parsed like other back-slashes, where it removes
the special meaning of the following character (e.g. 's/x/\//' replaces
an 'x' with a '/' ).

The previous behaviour lead to inconsistencies, exactly
as you've encountered. Compare:

   # '\t' means TAB, backslash affects the character that follows:
   $ echo x | sed-4.2.2 's/x/\t/' | hexdump -c
   0000000  \t  \n
   0000002

   # but here the '\c\' is taken as one item, and 't' is parsed by itself:
   $ echo x | sed-4.2.2  's/x/\c\t/' | hexdump -c
   0000000 034   t  \n
   0000003

   # yet '/' immediately after '\c\' was rejected:
   $ echo x | sed-4.2.2 -e 's/x/\c\/' | hexdump -c
   sed: -e expression #1, char 8: unterminated `s' command

Commit v4.2.2-99-g156e099 [1] fixed this behaviour.
To use backslash as control character, TWO backslashes are required -
just like using a literal backslash anywhere else:

   ## Two backslashes are needed for CTRL-\
   $ echo x | sed-4.4 's/x/\c\\/' | hexdump -c
   0000000 034  \n
   0000002

   ## A single backslash is not enough:
   $ echo x | sed-4.4 's/x/\c\/' | hexdump -c
   sed: -e expression #1, char 8: unterminated `s' command


   ## Ambigious usage is rejected:
   $ echo x | sed-4.4 's/x/\c\t/' | hexdump -c
   sed: -e expression #1, char 9: recursive escaping after \c not allowed

This behaviour was introduced in sed-4.3.

As such I'm closing this bug, but discussion can continue
by replying to this thread.

regards,
- assaf


[1] https://git.savannah.gnu.org/cgit/sed.git/commit/?id=156e0998

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