A regular expression input formatter that allows old value instead of clearing them

formatter.dart

/// [Author] Alex (https://github.com/AlexV525)
/// [Date] 2022/03/14 11:59
import 'packages:flutter/services.dart';

/// A formatter that takes a [RegExp] to match the value.
///
/// Currently the formatter only support match the first group.
class RegExpFormatter extends TextInputFormatter {
  RegExpFormatter(this.regExp);

  final RegExp regExp;

  @override
  TextEditingValue formatEditUpdate(
    TextEditingValue oldValue,
    TextEditingValue newValue,
  ) {
    if (newValue.text.isEmpty ||
        !newValue.composing.isCollapsed ||
        !newValue.selection.isCollapsed) {
      return newValue;
    }
    final Iterable<RegExpMatch> matches = regExp.allMatches(newValue.text);
    if (matches.length == 1 &&
        matches.first.group(0).toString() == newValue.text) {
      return newValue;
    }
    return oldValue;
  }
}