how to use startWith and substring in flutter

Question

@override
  Widget build(BuildContext context) {
    final AppStateManager manager = AppStateManager.of(context);

    String textLetter = manager.appState.replacementsController.text;
    String textCodeElec = manager.appState.replacementsController.value.text;
    // log('data: ${manager.appState.textEditingDeltaHistory.length}');

    

    if (textLetter.toLowerCase().startsWith('ap')) {
      textCodeElec += ' 01';
      textLetter = textLetter.substring(2);
    }
    
    if (textLetter.toLowerCase().startsWith('uyên')) {
      textCodeElec += ' 02';
      textLetter = textLetter.substring(2);
    }
    if (textLetter.toLowerCase().startsWith('oa')) {
      textCodeElec += ' 98';
      textLetter = textLetter.substring(2);
    }

    if (textLetter.toLowerCase().startsWith('áo')) {
      textCodeElec += ' 9409';
      textLetter = textLetter.substring(1);
    }
    if (textLetter.toLowerCase().startsWith('ch')) {
      textCodeElec += ' 44';
      textLetter = textLetter.substring(1);
    }
    if (textLetter.toLowerCase().startsWith('ng')) {
      textCodeElec += ' 34';
      textLetter = textLetter.substring(1);
    }

   

    log('sub1: ${textLetter}');
    // log('sub2: ${textLetter.substring(1)}');
    return Column(
      children: [
        _buildTextEditingDeltaViewCode(textCodeElec.trim()),// "nguyên choa" = 34 02 44 98
        _buildTextEditingDeltaViewHeader(),
        Expanded(
          child: ListView.separated(
            padding: const EdgeInsets.symmetric(horizontal: 35.0),
            itemBuilder: (context, index) {
              return _buildTextEditingDeltaHistoryViews(
                  manager.appState.textEditingDeltaHistory)[index];
            },
            itemCount: manager.appState.textEditingDeltaHistory.length,
            separatorBuilder: (context, index) {
              return const SizedBox(height: 2.0);
            },
          ),
        ),
        const SizedBox(height: 10),
      ],
    );
  }

I want the textfield to display as a sentence but it doesn't seem to be what I expected when I type "Nguyên Choa" it only shows 34 02 what I expected is "Nguyên Choa" = 34 02 44 98.I hope you experts can help me solve this difficult problem. I am truly grateful.