string str = @"<a href=""http:www.csdn.net"" target=""_blank"">csdn</a>
<a href=""http:www.csdn.net"">csdn</a>
<a href=""http:www.csdn.net"" target=""csdn"">csdn</a>
<a href=""http:www.csdn.net"" target=""baidu"">csdn</a>
<a target=""baidu"" href=""http:www.csdn.net"">csdn</a>
<a target=""_blank"" href=""http:www.csdn.net"">csdn</a>
<a href=""http:www.csdn.net"" target=""_blank"">csdn</a>";
            Regex reg = new Regex(@"(?is)(?<=<a\b)(?:((?:(?!target)[^>])*)>|[^>]*?target=""(?!_blank)[^'""\s]+""[^>]*?>)");
            string result = reg.Replace(str, delegate(Match m)
                {
                    if (m.Groups[1].Success)
                        return m.Groups[1].Value + @" target=""_blank""";
                    else
                        return Regex.Replace(m.Value, @"(?<=target="")[^""]+(?="")", "_blank");
                });
            Console.WriteLine(result);
/*
<a href="http:www.csdn.net" target="_blank">csdn</a>
<a href="http:www.csdn.net" target="_blank"csdn</a>
<a href="http:www.csdn.net" target="_blank">csdn</a>
<a href="http:www.csdn.net" target="_blank">csdn</a>
<a target="_blank" href="http:www.csdn.net">csdn</a>
<a target="_blank" href="http:www.csdn.net">csdn</a>
<a href="http:www.csdn.net" target="_blank">csdn</a>
*/