I noticed that to do syntax checking we have to put regex fields into the required fields list. This is a little bit awkward as I would like to do some syntax checking also on fields that could be empty.
Example: a birthday field that is not mandatory, but needs its syntax checked if values are entered.

So it would be better to have two lists:
1) one for the required fields
2) one for the fields that support regular expressions (regex).

This is also necessary if you write to a csv file as a user could enter the delimiter character of the csv file into a field.