A regex field is a "regular expression" field, which means a field that will only allow answer that match a certain pattern.
The IBAN field, for instance, a regex field, that will only allow answers that match the pattern XX00 0000 0000 0000.
The syntax to set up a regex field is quite complex, and we can set them up for you. Please contact your CSM or support if you wish to use a regex field in your form.
We're always happy to help ! :)