Negating regular expressions in filter expressions results in "internal error"

Hi,
when playing with filters I tried to use a simple regular expression filter “not regexp s.*” to filter my hosts. Unfortunately I received an internal error:

Did I do something wrong? In the end I would like to create filter expression that will give me only those hosts, that don’t have any tag assigned. There I thought about applying a negated regular expression on the hosts’ tag attribute.