flutter string add , for 1000
RegExp reg = RegExp(r'(\d{1,3})(?=(\d{3})+(?!\d))');
String Function(Match) mathFunc = (Match match) => '${match[1]},';
List<String> tests = [
'0',
'10',
'123',
'1230',
'12300',
'123040',
'12k',
'12 ',
];
for (String test in tests) {
String result = test.replaceAllMapped(reg, mathFunc);
print('$test -> $result');
}