isPin function

String? isPin(
  1. String val
)

return an error message if value does not pass the validation validation requires val: not equal null and length must be more or equal to 4 characters

Implementation

String? isPin(String val) {
  final RegExp reg = RegExp(r"^[0-9]{4}$");
  return reg.hasMatch(val) ? null : "Pin must be a 4 digit number";
}