Returns nil instead of regex match group inside conditional

Problem with DBI gem under jruby-9.0.0.0.pre

``` ruby
def parse_url(driver_url)
  if driver_url =~ /^(DBI|dbi):([^:]+)(:(.*))$/ 
    [$2, $4]
  else
    raise InterfaceError, "Invalid Data Source Name"
  end
end

parse_url("dbi:jdbc:sqlite")
```

returns [nil, nil] instead of ["jdbc", "sqlite"]
