Test bare \b and \B in regexes are forbidden

RT#128986: https://rt.perl.org/Ticket/Display.html?id=128986
Fix commits:
rakudo/rakudo@08589d32fa
Raku/nqp@66308ea63d
Raku/nqp@c5f38888ec

S05-metasyntax/regex.t

@@ -1,7 +1,7 @@
 use v6;
 use Test;
 
-plan 42;
+plan 47;
 
 throws-like 'qr/foo/', X::Obsolete, 'qr// is gone';
 
@@ -150,4 +150,12 @@ throws-like 'Regex.new.perl', Exception, '"Regex.new.perl dies but does not segf
 ok 'a' ~~ /a:/, '/a:/ is a valid pattern and matches a';
 ok 'a' ~~ /a: /, '/a: / is a valid pattern and matches a';
 
+{ # RT #128986
+    throws-like '/\b/', X::Obsolete, 'bare \b is no longer supported';
+    throws-like '/\B/', X::Obsolete, 'bare \B is no longer supported';
+    ok "\b" ~~ /"\b"/, '\b still works when quoted';
+    ok "\b" ~~ /<[\b]>/, '\b still works in character class';
+    is ("a\bc" ~~ m:g/<[\B]>/).join, 'ac', '\B still works in character class';
+}
+
 # vim: ft=perl6