正则表达式前3位数字，左移

Question

I need to create an edit mask for a textbox that only allows digits 0-9 (on it's own, easy enough!), but must always be 3 digits long, and left padded with 0's if a number such as 69 is entered. It is used for a text box holding azimuth values, so I want a bearing such as 78 to show as 078 as the control loses focus.

The following has been suggested in another thread but that is not my requirement:

someText = Regex.Replace(someText, @"\d+", n => n.Value.PadLeft(8, '0'));

At the moment I do the following in the Validated event, but I have feeling a RegEx edit mask may be more elegant:

textBox.Text = textBox.Text.PadLeft(3, '0');

Answer 1

You can use a MaskedTextBox as well with 000 mask.

Mask
Gets or sets the input mask to use at run time.

And MaskedTextBox.Mask Property reference :

Masking element Description
0 Digit, required. This element will accept any single digit between 0 and 9.
9 Digit or space, optional.
# Digit or space, optional. If this position is blank in the mask, it will be rendered as a space in the Text property. Plus ( + ) and minus ( - ) signs are allowed.

There are more options there.

Also, you can set the PromptChar to 0 to make it look nice (as if padded).

EDIT:

I have just tried to create a function for TextChanged event, to achieve what you want without masked text box. Just for an exercise:

private void textBox2_TextChanged(object sender, EventArgs e)
{
    var sel = textBox2.SelectionStart;
    if (string.IsNullOrWhiteSpace(textBox2.Text))
    {
        textBox2.Text = "000";
        return;
    }
    if (sel != 0)
    {
        var symbol_entered = textBox2.Text.Substring(sel - 1, 1);
        if (!Char.IsDigit(symbol_entered.ToCharArray()[0]))
        {
            sel--;
            textBox2.Text = textBox2.Text.Remove(sel, 1);
        }
        else
        { // entered a digit
            if (textBox2.Text.Length == 4 && textBox2.SelectionStart == 4) // last digit entered 
                textBox2.Text = textBox2.Text.Substring(0, 3).PadLeft(3, '0'); // trim it
            else if (textBox2.Text.Length == 4)
                textBox2.Text = textBox2.Text.Remove(sel, 1);
         }
    }
    if (!string.IsNullOrWhiteSpace(textBox2.Text)) // removed the first digit
    {
        textBox2.Text = textBox2.Text.PadLeft(3, '0'); // trim it
        textBox2.SelectionStart = sel;
    }
}

Have fun! :)