The problem with the regex was the expression ":b*" and ":b@". Previously this worked and it is currently broken (simple fix).
Having said that, :b was really a bad idea for a built-in regex because it gets incorrectly used. Too bad is too late to change that. It's being incorrectly used in this regex. :b actually is [ \t]# and not just [ \t]. Putting a loop around a loop is very bad. You really don't want ([ \t]#)*. It makes the regex engine do way to may tries.