Flutter add separator character after given interval in TextFormField

I want have a TextFormField that formats the input depending on a given separator and interval.

E.g. separator = '-' and interval = 4 the output should be the following:

Input: 1234 Output: 1234

Input 12345678 Output: 1234-5678

Input: 12345678910 Output: 1234-5678-910

The separator should be inserted right when the user types in the 5th character in this example.

I tried multiple things now, but can not get a general working solution. That was my last try:

  void _addSeperator(String value) {
    final separator = widget.separator!;
    final separatorInterval = widget.separatorInterval!;

    value = value.replaceAll(separator, ''); // Remove existing separators
    int length = value.length;
    int separatorCount = (length / separatorInterval).floor();
    for (int i = 1; i <= separatorCount; i++) {
      int index = i * separatorInterval +
          (i - 1); // Calculate the index to insert separator
      if (index < length) {
        value = value.substring(0, index) +
            separator +
            value.substring(index, value.length);
      }
    }

    _controller.text = value;
  }

I thought this shouldnt be that hard, but I can not make it work. Also didn't find anything on this. Let me know if you need more info.

I got it working with this:

class UISeparatorInputFormatOptions {
  final String separator;
  final int interval;

  UISeparatorInputFormatOptions({
    required this.separator,
    required this.interval,
  });
}

class UISeparatorInputFormatter extends TextInputFormatter {
  /// IMPORTANT: String must not contain the options.separator
  /// otherwise this InputFormatter will not work correctly.
  final UISeparatorInputFormatOptions options;

  UISeparatorInputFormatter({
    required this.options,
  });

  @override
  TextEditingValue formatEditUpdate(
    TextEditingValue oldValue,
    TextEditingValue newValue,
  ) {
    // Remove existing separators
    String newText = newValue.text.replaceAll(
      options.separator,
      '',
    );

    StringBuffer buffer = StringBuffer();
    for (int i = 0; i < newText.length; i++) {
      if (i > 0 && i % options.interval == 0) {
        buffer.write(options.separator);
      }
      buffer.write(newText[i]);
    }

    String formattedString = buffer.toString();
    return newValue.copyWith(
      text: formattedString,
      // This ensures that the cursor follows the text properly
      selection: TextSelection.collapsed(offset: formattedString.length),
    );
  }
}