Parser Combinator requires every regex is initialized before usage

parser.scala

import scala.util.parsing.combinator._

import scala.util.matching.Regex.Match
import scala.util.matching.Regex
import scala.annotation.{switch, tailrec}

object T extends RegexParsers  {

  abstract class AbstractSelector[S <: AbstractSelector[S]]


  class ComplexSelector(val combinator: Char, val x: CompoundSelector, val xs: ComplexSelector)
    extends AbstractSelector[ComplexSelector]() {

    override def toString = xs match {
      case _: ComplexSelector => xs + combinator.toString + x
      case _ => x.toString
    }
  }

  case class CompoundSelector(tpe: String, simpleSelectors: List[SimpleSelector])
    extends AbstractSelector[CompoundSelector] {

    override def toString = tpe + simpleSelectors.mkString("")
  }

  class SimpleSelector extends AbstractSelector[SimpleSelector]

  override def skipWhitespace = false


  private final val Combinator = """\s*[+>~]\s*""".r ^^ {
    case cmb => (cpx: ComplexSelector, cpd: CompoundSelector) =>
      new ComplexSelector(cmb.trim.head, cpd, cpx)
  }
  private final val Whitespace = """\s+""".r ^^ {
    case cmb => (cpx: ComplexSelector, cpd: CompoundSelector) =>
      new ComplexSelector(' ', cpd, cpx)
  }
  private final val ComplexZero = CompoundParser ^^ {
    case cpd =>
      new ComplexSelector('\u0000', cpd, null)
  }

  def ComplexParser: Parser[ComplexSelector] =
    chainl1(ComplexZero, CompoundParser, (Combinator|Whitespace))

  def CompoundParser: Parser[CompoundSelector] = (ident|"*")  ^^ {
    case tpe =>
      CompoundSelector(tpe, List())
  }

  private final val ident = "[0-9A-Za-z_-]+".r
  def main(args: Array[String]): Unit =
    println(ident.toString)
    println(parseAll(ComplexParser, "div").get)
}

I got it.

https://github.com/scala/scala-parser-combinators/blob/master/src/main/scala/scala/util/parsing/combinator/Parsers.scala#L248

By implicit conversion, ident is a Parser[String], which has plenty of call by name parameter and ... say, null value filled val get stuffed into those method's apply closure (yes, tons of closure). So the val should be lifted up.