URL validation in C# window application

private void TextBox1_Validating(object sender, CancelEventArgs e)
        {
            System.Text.RegularExpressions.Regex rURL = new System.Text.RegularExpressions.Regex(
            @"^([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}" +
            @"\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\" +
            @".)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$");
            if (txtURL.Text.Length > 0)
            {
                if (!rURL.IsMatch(txtEmail.Text))
                {
                    MessageBox.Show("Please provide valid URL", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                    txtURL.SelectAll();
                }
            }
        }