Re: AS_DIRNAME and expr

[David Morgan]

Hi Paul,

Paul Eggert wrote:
> 
> > From: David Morgan <address@hidden>
> > Date: Thu, 09 Nov 2000 08:12:01 -0800
> 
> > > | > expr 'a' : '\(b\)' && echo failure
> > > |
> > > | Gives:
> > > | 0
> > >
> > > Result is wrong, $? right.
> 
> > Wrong explanation - no characters match so the output of 0 is correct
> > (At least I think so)
> 
> The POSIX standard says:
> 
>    Usually, the matching operator shall return a string representing
>    the number of characters matched ('0' on failure). Alternatively,
>    if the pattern contains at least one regular expression
>    subexpression "[\(...\)]", the string corresponding to "\1" shall
>    be returned.
> 
> This leaves it ambiguous as to what expr 'a' : '\(b\)' should return.
> One could easily argue that it should be '0'.  But in practice,
> GNU/Linux and Solaris expr both return the empty string, under the
> theory that the operation is returning a string.

I hadn't realized the ambiguity.

I can test this on some other boxes and see what happens.
> 
> However, autoconf's use of expr in AS_DIRNAME should be OK either way,
> so I don't see why this matters to autoconf.
> 
> > > Man!  Did I miss something?  David, could you double check this is
> > > right?  Paul, what do you think about this?
> 
> Sorry, I've lost context.  Why is this a problem?  If expr exits with
> nonzero status, then AS_DIRNAME will fall back on sed, right?

It will use the output of the expr and then append the output of the 
sed.  So we'd get a 0 . or something similar.

(Ignoring the other problem I found with the sed expression - missing a -p 
is what someone said)

Does that make sense?

Regards
 David