How to handle a superclass constructor call with a complex condition?

As my Scala learning exercise, I'm writing a little library to allow
regular expressions to be dealt with at a more abstract level. One of
the classes I have is this short one:

protected case class RawCharSet(in:String, notIn:String) extends
Matcher("[" + in + (if (notIn =="") "" else "&&[^" + notIn + "]")
+ "]")

Basically, it produces regular expressions like [abc] if the second
argument is "", or [abc&&[^def]] if the second argument is a value
such as "def".

However, this is less than readable. I'd like to factor it into a form
where the the string argument is built first, and then the constructor
(or superclass constructor, with the code organization above) is
called--but that's not possible, as even for auxiliary constructors,
the first statement must be a call on "this(...)".

Any tricks I'm not aware of that you can suggest?

Thanks,
Ken