format method
AADHAAR format is 9999 9999 9999
So add space after every 4 digits.
Implementation
String? format(String? aadhaar) {
if (aadhaar == null) return null;
// remove the spaces from aadhaar
aadhaar = aadhaar.replaceAll(RegExp(r"\s+"), "");
return aadhaar
.replaceAllMapped(RegExp(r".{4}"), (match) => "${match.group(0)} ")
.trim();
}