Only allow specific characters in textbox

Question

How can I only allow certain characters in a Visual C# textbox? Users should be able to input the following characters into a text box, and everything else should be blocked

Answer 1

Intercept the KeyPressed event is in my opinion a good solid solution. Pay attention to trigger code characters (e.KeyChar lower then 32) if you use a RegExp.

But in this way is still possible to inject characters out of range whenever the user paste text from the clipboard. Unfortunately I did not found correct clipboard events to fix this.

So a waterproof solution is to intercept TextBox.TextChanged. Here is sometimes the original out of range character visible, for a short time. I recommend to implement both.

using System.Text.RegularExpressions;

private void Form1_Shown(object sender, EventArgs e)
{
    filterTextBoxContent(textBox1);
}


string pattern = @"[^0-9^+^\-^/^*^(^)]";

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    if(e.KeyChar >= 32 && Regex.Match(e.KeyChar.ToString(), pattern).Success) { e.Handled = true; }
}

private void textBox1_TextChanged(object sender, EventArgs e)
{
    filterTextBoxContent(textBox1);
}

private bool filterTextBoxContent(TextBox textBox)
{
    string text = textBox.Text;

    MatchCollection matches = Regex.Matches(text, pattern);
    bool matched = false;

    int selectionStart = textBox.SelectionStart;
    int selectionLength = textBox.SelectionLength;

    int leftShift = 0;
    foreach (Match match in matches)
    {
        if (match.Success && match.Captures.Count > 0)
        {
            matched = true;
            Capture capture = match.Captures[0];

            int captureLength = capture.Length;
            int captureStart = capture.Index - leftShift;
            int captureEnd = captureStart + captureLength;

            int selectionEnd = selectionStart + selectionLength;

            text = text.Substring(0, captureStart) + text.Substring(captureEnd, text.Length - captureEnd);

            textBox.Text = text;

            int boundSelectionStart = selectionStart < captureStart ? -1 : (selectionStart < captureEnd ? 0 : 1);
            int boundSelectionEnd = selectionEnd < captureStart ? -1 : (selectionEnd < captureEnd ? 0 : 1);

            if (boundSelectionStart == -1)
            {
                if (boundSelectionEnd == 0)
                {
                    selectionLength -= selectionEnd - captureStart;
                }
                else if (boundSelectionEnd == 1)
                {
                    selectionLength -= captureLength;
                }
            }
            else if (boundSelectionStart == 0)
            {
                if (boundSelectionEnd == 0)
                {
                    selectionStart = captureStart;
                    selectionLength = 0;
                }
                else if (boundSelectionEnd == 1)
                {
                    selectionStart = captureStart;
                    selectionLength -= captureEnd - selectionStart;
                }
            }
            else if (boundSelectionStart == 1)
            {
                selectionStart -= captureLength;
            }

            leftShift++;
        }
    }

    textBox.SelectionStart = selectionStart;
    textBox.SelectionLength = selectionLength;

    return matched;
}