>> The bug: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=6286.

And here's a tentative patch for the last example (regexp as a first
parameter in a paren-less method invocation).

It adds a check whether the regexp is followed by a comma or a
block, though, so the full example should look like this:

Given /A user is logged in/ do
  # whatever
end

AFAIK, all Cucumber step definitions need a block (that's where the
definition goes), so it shouldn't be a problem.