As a point of information, it does happen for gawk.
Could you please say where that happens? I just now looked at the gawk trunk, and the only two places I saw it calling dfaexec (helpers/testdfa.c and re.c), it passed a nonnull backref argument.
Obviously this revert does reintroduce the bug with [\]-\]] that you mentioned with Awk regular expressions. It reintroduces similar bugs for GNU grep. At the very least we should fix that. I'll follow up with a patch.
