A "Replace" option should be available in the "Regular Expression" action. Now only "Check match", "Split" and "Extract" are available. Why is that? It makes it impossible to replace "forbidden" characters in a form before submitting. The available rules does not help either…
Apologies for the delay in responding, glad you've found the empty-variable solution already!
Please do reach out if you have any other questions, or feedback.
Regards,
Kate