I need to extract domain (`four.five`) from URL (`one.two.three.four.five`) in a Lua string variable.

I can't seem to find a function to do this in Lua.

EDIT: 

By the time the URL gets to me, the `http` stuff has already been stripped off. So, some examples are: 

```
a) safebrowsing.google.com 
b) i2.cdn.turner.com 
c) powerdns.13854.n7.nabble.com 

```

so my result should be: 

```
a) google.com
b) turner.com
c) nabble.com

```