On further investigation, it is debatable whether the other failures
are errors. (Please ignore the rest of the email if this has been
talked to death).
The regexps in question all deal with variations of \b on empty or one
space strings. For example
(match-string "\\b" "") ==> 0 but the test expects nil.
If we accept that (string-match "" "") ==> 0, which currently both
Emacs and XEmacs agree on, then you could argue that the match is at
the start of a empty word.
Now my perl is very weak, but I though perl would give an alternative
regexp implementation. Please correct me if any of the following is
Perl seems to agree that (string-match "" "") ==> 0
$text = "";
$text =~ // && print "OK\n";
$text =~ /\b/ && print "OK\n";