A simple text input formatter for grouping text by splitAt and separating with the separator e.g. 2222-1111 has splitAt: 4 and seperator: '-'

card_input_formatter.dart

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

class CardInputFormatter extends TextInputFormatter {
  CardInputFormatter({required this.separator, this.splitAt = 4});
  final String separator;
  final int splitAt;

  @override
  TextEditingValue formatEditUpdate(
    TextEditingValue oldValue,
    TextEditingValue newValue,
  ) {
    final oldText = oldValue.text;
    final newText = newValue.text;
    var endsWithSeparator = false;

    // if you add text
    if (newText.length > oldText.length) {
      for (final char in separator.characters) {
        if (newText.substring(0, newText.length - 1).endsWith(char)) {
          endsWithSeparator = true;
        }
      }
      final clean = newText.replaceAll(separator, '');
      if (!endsWithSeparator &&
          clean.length > 1 &&
          clean.length % splitAt == 1) {
        return newValue.copyWith(
          text: newText.substring(0, newText.length - 1) +
              separator +
              newText.characters.last,
          selection: TextSelection.collapsed(
            offset: newValue.selection.end + separator.length,
          ),
        );
      }
    }

    // if you delete text
    if (newText.length < oldText.length) {
      for (final char in separator.characters) {
        if (oldText.substring(0, oldText.length - 1)
            .endsWith(char)) {
          endsWithSeparator = true;
        }
      }
      final clean = oldText.substring(0, oldText.length - 1).replaceAll(
            separator,
            '',
          );
      if (endsWithSeparator &&
          clean.isNotEmpty &&
          clean.length % splitAt == 0) {
        return newValue.copyWith(
          text: newText.substring(
            0, 
            newText.length - separator.length,
          ),
          selection: TextSelection.collapsed(
            offset: newValue.selection.end - separator.length,
          ),
        );
      }
    }

    return newValue;
  }
}