Asp.net实现文章内关键词替换

Regex reg = new Regex(textBox1.Text, RegexOptions.None);
            textBox4.Text = reg.Replace(textBox3.Text, textBox2.Text);            string content = textBox3.Text;
            string content2 = content;            List keywordlist = new List();
            keywordlist.Add("网上订餐");
            keywordlist.Add("订餐");
              foreach (string keyword in keywordlist)
            {
                string lnkKeyword = "" + keyword + "";
                reg = new Regex(keyword, RegexOptions.Compiled | RegexOptions.IgnoreCase);
                while (true)
                {
                    Match m = reg.Match(content2);                    
                    if (m.Success)
                    {
                        int start = m.Index;
                        int length = keyword.Length;
                        content = content.Substring(0, start) + lnkKeyword + content.Substring(start + length);
                        content2 = content2.Substring(0, start) + new string('*', lnkKeyword.Length) + content2.Substring(start + length);
                    }
                    else
                    {
                        break;
                    }
                }
            }            textBox4.Text = content ;