Programmation xlsform

Support
1

how I can put a constraint to link two fields with the same data example: mail 1: peter@gmail.com and we ask a person to repeat if mail 2 is different from mail1 you cannot go to the next screen

2

If you have two questions and you need to make sure the answers are equal you can add something like (to the row with mail2):
. = ${mail1}

3

What if I want to limit a field to only 4 digits

4

There is string-length function: https://docs.getodk.org/form-operators-functions/#string-length