I have a number value in `string` as

```
string strNum = "12345678.90";

```

I want to format it with comma separator using `regex` in `String.Format()`

On using `"{0:n0}"` format in 

```
String.Format("{0:n0}", Convert.ToDouble(strNum)); 

```

it is giving me output as `"12,345,679"`

Instead of this i want output as `"1,23,45,678.90"`. After thousand's place i want comma separator after 2 digits each for lakhs, crores and so on

How can this be achieved?