Matematico Web Game in ASP.NET using RegEx

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class OneOf25 : System.Web.UI.UserControl
    {
    
    public  string Card
        {
        get { return (ViewState["Card"] == null)? "" : (string)ViewState["Card"]; }
        set { ViewState["Card"] = value; }
        }

    public delegate void OnClickEventHandler(object sender, EventArgs e);
    public event OnClickEventHandler OnClick;

    
    protected void Page_PreRender(object sender, EventArgs e)
        {
        CardButton.Visible = (Card.Equals(""));
        CardImage.Visible = !(Card.Equals(""));
        if (CardImage.Visible) {
            CardImage.ImageUrl = "~/Cards/" + Card + ".gif";
            }
        }


    protected void CardButton_Click(object sender, EventArgs e)
        {
        if (OnClick != null) OnClick(this, e);
        }
}