Re: autoconf doc fixes for `...` gotchas

[Paul Eggert]

Ralf Wildenhues <address@hidden> writes:

>>      (Limitations of Builtins): Use expr "X...", not expr "x...", as
>>      X insulates us from future changes to Posix.
>
> Could you be bothered to elaborate what those changes are?

In case POSIX adds a new expr operator starting with "x".
(It's kind of theoretical.)

> If you do this, please also do this (and maybe note the significance of
> X over x):

It's probably not worth worrying users about X over x, but I did change
it to X.  Thanks for mentioning it.