Well that's because I've never used BNF before and I don't really know what I'm doing with it.

As for the special chars, they are not used in aXML, only in the Perl plugins. They exist solely so Perl can correctly work with aXML. aXML itself does not need them or care about them at all, hence they don't belong in a formal specification of aXML.

Don't make excuses; fix it.

I don't really know what I'm doing with it

You're specifying your language's syntax.

Well that's because I've never used BNF before

No, you didn't have any problem with BNF. The problem is that you didn't describe aXML.

As for the special chars, they are not used in aXML, only in the Perl plugins.

Are you back to saying one has to use

sub escape {
   my ($s) = @_;
   for ($s) {
      die "\"$1\" can't be produced by aXML safely"
         if /([<>()\[\]])/;
      return $_;
  }
}
[download]

instead of

sub escape {
   my ($s) = @_;
   for ($s) {
      die "\"$1\" can't be produced by aXML safely"
         if /(&(?:lab|rab|lcb|rcb|lsb|rsb);)/;
      s/\</&lab;/g;
      s/\>/&rab;/g;
      s/\(/&lcb;/g;
      s/\)/&lcb;/g;
      s/\[/&lsb;/g;
      s/\]/&rsb;/g;
      return $_;
  }
}
[download]

If it's not builtin, then a plugin can't rely on the person using aXML having provided a plugin to handle &lab;, which means you can't use anyone else's plugins, which means you can't reuse code, which means your comparison with TT was an outright lie.