Re: [PATCH 1/3] Properly expand @configure_input@ in config.status.

[Paolo Bonzini]

> But it doesn't work the way it should.  Sorry, I should have given an
> example:  With GNU sed 4.1.5,
>   echo x | sed 's&x&ab\&c&'
>
> results in
>   abxc
>
> rather than
>   ab&c
>
> which is what is returned by the seds on AIX, BSDs, Solaris, IRIX,
> Tru64, and reading SUSv3, I think it does not specify which is right.

Yes, it is undefined and the current behavior is more coherent with what 
is done on the LHS of the `s' command.  There, you want to strip slashes 
or the following command

   s/a\/b//

will rely on the regex matcher matching a literal slash for the escape 
\/.  This behavior changed in 4.1.x as part of a general reorganization 
of the sed parser to better support 7-bit multibyte character sets (e.g. 
ISO-2022) in the sed commands.  In the command above, sed sees

   s&x&ab\&c&

as if it was:

   s/x/ab&c/

The same behavior applies to the LHS: backslashed items are *not* passed 
down to the regex matcher.  For example, in

   s|a\|b||

older (up to 4.0.x) GNU seds will parse \| as a special escape 
(alternation) but 4.1.x and non-GNU seds will parse it as a literal pipe.

Paolo