Group Creation Empty keyword

Hello folks,
I want to create the grouping by readstatus shown on the JabRef hompage and docs.

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I already created a static group By Status and the two groups Skimmed and Read using “Collect by searching for a keyword” which are working

What troubles me is how to set up a group that selects entries with an empty readstatus field.
I tried to use a regular expression ^$ which should select empty strings but that doesn’t work.

Any ideas?
Thanks!

I haven’t found a solution yet. Has someone else tried to achieve this ?