TStringGrid Input Validation to permit only numbers, decimals, and commas on C++ Builder XE8

This question is an extension of another question:

TEdit Input Validation on C++ Builder XE8

I have an editable TStringGrid. I only want the user to type numbers and a maximum of one decimal point or comma for each cell in the grid.

From the above link, I understand how to permit only certain keys, but not how to keep track of how many times a certain key-value already exists in the given cell.

From the above link, I have this:

void __fastcall TSetDataForm::ProbabilityGridKeyPress(TObject *Sender, System::WideChar &Key)
{
    if( Key == VK_BACK )
        return;

    if( (Key < L'0') || (Key > L'9') )
    {
        ShowMessage("Please enter numerals only");
        Key = 0;
    }
}

How do I allow '.' or ',' but only once?

I would suggest using TryStrToFloat() to validate the input, then there is no question whether the user is entering a valid decimal string or not. You would just need to handle the extra cases where:

• the user is typing a character somewhere other than the end of the edit field, with or without text selected.

• the user is copy/pasting text into the editor.

For example:

class TStringGridAccess : public TStringGrid
{
public:
    __property InplaceEditor;
};

void __fastcall TSetDataForm::ProbabilityGridKeyPress(TObject *Sender, System::WideChar &Key)
{
    switch (Key)
    {
        case 3: // Ctrl-C
        case 8: // Backspace
            return;

        case 22: // Ctrl-V
        {
            Key = 0;

            TInplaceEdit *Editor = ((TStringGridAccess*)ProbabilityGrid)->InplaceEditor;

            String SaveCellText = ProbabilityGrid->Cells[ProbabilityGrid->Col][ProbabilityGrid->Row];

            String SaveEditText = Editor->Text;
            int SaveSelStart = Editor->SelStart;
            int SaveSelLen = Editor->SelLength;

            Editor->Perform(WM_PASTE, 0, 0);

            TFormatSettings fmt = TFormatSettings::Create();
            fmt.DecimalSeparator = _D('.');

            double value;
            if (TryStrToFloat(Editor->Text, value, fmt))
                return;

            ProbabilityGrid->Cells[ProbabilityGrid->Col][ProbabilityGrid->Row] = SaveCellText;

            Editor->Text = SaveEditText;
            Editor->SelStart = SaveSelStart;
            Editor->SelLength = SaveSelLen;

            break;
        }

        case _D('0'):
        case _D('1'):
        case _D('2'):
        case _D('3'):
        case _D('4'):
        case _D('5'):
        case _D('6'):
        case _D('7'):
        case _D('8'):
        case _D('9'):
        case _D('.'):
        {
            TInplaceEdit *Editor = ((TStringGridAccess*)ProbabilityGrid)->InplaceEditor;

            String str = Editor->Text;
            int idx = Editor->SelStart;
            int len = Editor->SelLength;

            String str2 = str.SubString(1, idx) + Key + str.SubString(1+idx+len, MaxInt);

            TFormatSettings fmt = TFormatSettings::Create();
            fmt.DecimalSeparator = _D('.');

            double value;
            if (TryStrToFloat(str2, value, fmt))
                return;

            break;
        }
    }

    ShowMessage(_D("Please enter decimals only"));
    Key = 0;
}