How to validate time using regex?

validation on given time value using regex...

//Here user can enter in : HH:MM or HH:MM:SS or HH:MM:SS:mmm

//In 12 Hour format...

private void button1_Click(object sender, EventArgs e)
{
           System.Text.RegularExpressions.Regex regex = new System.Text.RegularExpressions.Regex(@"^([0-9]|1[0-2]):[0-5]\d(:[0-5]\d(\:\d{1,3})?)?$");
            if (regex.IsMatch(textBox1.Text))
            {
                 MessageBox.Show("Time is correct.");
            }
            else
            {
                 MessageBox.Show("Time is not correct.");
            }
}