使用DatePicker代替键盘，以确保格式正确。

您真的应该使用DatePicker：What is the correct way to add date picker in flutter app?
如果你坚持用非标准的方式来做，你可以使用FilteringTextInputFormatter：Flutter - Regex in TextFormField

您可以将其用作inputFormatters，将TextInputType.number用作keyboardType。

class DateTextFormatter extends TextInputFormatter {
  @override
  TextEditingValue formatEditUpdate(
      TextEditingValue oldValue, TextEditingValue newValue) {
    if (newValue.text.length > oldValue.text.length &&
        newValue.text.isNotEmpty &&
        oldValue.text.isNotEmpty) {
      if (RegExp('[^0-9/]').hasMatch(newValue.text)) return oldValue;
      if (newValue.text.length > 10) return oldValue;
      if (newValue.text.length == 2 || newValue.text.length == 5) {
        return TextEditingValue(
          text: '${newValue.text}/',
          selection: TextSelection.collapsed(
            offset: newValue.selection.end + 1,
          ),
        );
      } else if (newValue.text.length == 3 && newValue.text[2] != '/') {
        return TextEditingValue(
          text:
              '${newValue.text.substring(0, 2)}/${newValue.text.substring(2)}',
          selection: TextSelection.collapsed(
            offset: newValue.selection.end + 1,
          ),
        );
      } else if (newValue.text.length == 6 && newValue.text[5] != '/') {
        return TextEditingValue(
          text:
              '${newValue.text.substring(0, 5)}/${newValue.text.substring(5)}',
          selection: TextSelection.collapsed(
            offset: newValue.selection.end + 1,
          ),
        );
      }
    } else if (newValue.text.length == 1 &&
        oldValue.text.isEmpty &&
        RegExp('[^0-9]').hasMatch(newValue.text)) {
      return oldValue;
    }
    return newValue;
  }
}