Full width number to half width number

string_extension.dart

const _fullLengthCode = 65248;

extension StringExtension on String {
  String toHalfWidthNumber() {
    final regex = RegExp(r'^[０-９]+$');
    final string = runes.map<String>((rune) {
      final char = String.fromCharCode(rune);
      return regex.hasMatch(char)
          ? String.fromCharCode(rune - _fullLengthCode)
          : char;
    });
    return string.join();
  }
}