Group not matching yields null.
```
scala> val r = """(?:(.*)\.)?(.*)""".r
r: scala.util.matching.Regex = (?:(.*)\.)?(.*)

scala> "a.b.c.d" match { case r(x,y) => s"$x/$y" }
res0: String = a.b.c/d

scala> "d" match { case r(x,y) => s"$x/$y" }
res1: String = null/d
```