string viewScript = " left join (select T1 as T2 from Table group by T1 ) as t on t (select 1 from TT) as c";
viewScript = Regex.Replace(viewScript, @"(?i)as ", "");

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

string viewScript = " left join (select T1 as T2 from Table group by T1 ) as t on t (select 1 from TT) as c";

                viewScript = Regex.Replace(viewScript,@"(?<=join\s*?\([^)]+?\).*?)\s+as\s+"," ");
                // left join (select T1 as T2 from Table group by T1 ) t on t (select 1 from TT) c

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

            string source = "left join (select T1 as T2 from Table group by T1 ) as t on t (select 1 from TT) as c";
            Regex reg = new Regex(@"(?<=\([^\)]*?\) )as");
            source = reg.Replace(source, @"");

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

 string sql=@"string viewScript = "" left join (select T1 as T2 from Table group by T1 ) as t on t (select 1 from TT) as c";
 sql= Regex.Replace(sql, @"(?<=\)\s+)as", "");