string str = "<input type=\"text\" ......./>";
        Regex reg=new Regex(@"(?<=<input[^>]+?)(type=""text"")(?=[^>]+?>)");
        str = reg.Replace(str, "$1 Value=\"0\"");
        Response.Write( Server.HtmlEncode(str));
//输出：<input type="text" Value="0" ......./>

------解决方案--------------------
C# code

string s = "aaaaaaa<input type=\"text\" ......./>bbbbbbbbb";
string Result= Regex.Replace(s, @"<input\s+type=['""]text['""]\s+(?!value)", "<input type=\"text\" value=\"0\"");