Re: M4 syntax $11 vs. ${11}

[Paul Eggert]

Eric Blake <address@hidden> writes:

> Here's an updated (and smaller) patch that makes CVS autoconf happy with
> branch-1_4 of M4, after today's commit that adds --warn-syntax to M4.

In looking at this patch, I wonder: why not have M4 2.0 use the
extension only if the input is of the form ${DIGITSTRING}, where
DIGITSTRING is a sequence of one or more ASCII digits?  That way, we
needn't change Autoconf's m4 code at all (though the documentation
should still be changed, probably).

Very few shell scripts will contain usages like ${11} or ${0}, whereas
lots of them will have usages like ${1-'x'}.  If the M4 extension
uses only a syntax that is unlikely to occur in a shell script,
then we needn't worry about this issue so much.

I'm not so much worried about Autoconf proper here, as about macros in
other places (like Gnulib) that will also be affected by this change.