split

main.dart

void main() {
  var numCartao = splitIntoChunks(string: '3991641839424977', size: 4);
  print(numCartao);

  var numCartao2 = '3991641839424977'.splitIntoChunks(size: 4);
  print(numCartao2);
}

String splitIntoChunks({String string, int size, String separator = ' '}) {
  return string.splitMapJoin(
    RegExp(".{1,$size}"),
    onMatch: (Match m) => '${m.group(0)}$separator',
    onNonMatch: (String n) => n,
  );
}

extension SplitIntoChunks on String {
  String splitIntoChunks({int size, String separator = ' '}) {
    return this.splitMapJoin(
      RegExp(".{1,$size}"),
      onMatch: (Match m) => '${m.group(0)}$separator',
      onNonMatch: (String n) => n,
    );
  }
}