private string m_OldText;

        private void textBox1_KeyDown(object sender, KeyEventArgs e)
        {
            m_OldText = this.textBox1.Text;
        }

        private void textBox1_KeyUp(object sender, KeyEventArgs e)
        {
            if (e.KeyValue == 8)
            {
                if (!m_OldText.Contains(",") && !m_OldText.Equals(this.textBox1.Text))
                {
                    this.textBox1.Text = String.Empty;
                    return;
                }
                string[] oldParts = m_OldText.Split(',');
                string[] newParts = this.textBox1.Text.Split(',');
                if (oldParts.Length == newParts.Length)
                {
                    string value = String.Empty;
                    for (int i = 0; i < oldParts.Length; i++)
                    {
                        if (oldParts[i].Equals(newParts[i]))
                        {
                            if (oldParts[i].Length > 0)
                            {
                                value += oldParts[i];
                                value += ',';
                            }
                        }
                    }
                    this.textBox1.Text = value;
                }
                else
                {
                    this.textBox1.Text = m_OldText;
                }
            }
        }