It picks characters from the whole unicode range. Those strings should indeed match all regex constraints. You can add a constraint like: ^[\x21-\x7E]*$ T

It picks characters from the whole unicode range. Those strings should indeed match all regex constraints. You can add a constraint like: ^[\x21-\x7E]*$ To only choose printable ASCII characters. EDIT: no you're right, there is a bug. I would expect random unicode chars but not always the same box.