.net 中替换字符串的问题

Question

string test = "select * from testtable where id in(select id form table where tid=1)";
string _where = Regex.Replace(test, @"\ where\ ", " where (", RegexOptions.IgnoreCase | RegexOptions.Compiled);

我要得到的结果是 select * from testtable where (id in(select id form table where tid=1)

也就是说遇到的第一个where要替换成 where ( 应该怎么写那个正则啊？