Bug in AX_CHECK_POSIX_REGCOMP in current Autoconf Archive

The current AX_CHECK_POSIX_REGCOMP macro in Autoconf Archive calls this:

  return regcomp( &re, "|no.*", flags );
}]])],[ax_cv_posix_regcomp=yes],[ax_cv_posix_regcomp=no],[ax_cv_posix_regcomp=no]

But according to the regex(7) man page on Linux, branches in a POSIX
regex are required to be non-empty, so the regular expression "|no.*"
appears to be invalid since it conatins an empty branch.

And indeed, this macro fails on NetBSD, with regcomp returning 14, which
is the error code REG_EMPTY.

This is such a specific regex that I find it hard to believe that it's
just a coincidence that the test it's doing contains an empty branch.
Unfortunately, the Git history doesn't go back far enough to determine
why this is the way it is. I don't suppose you have any idea?

Interestingly, it doesn't fail on Linux.

Thanks,

Jonathan Kamens