http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1

acceptheaderparser.scala

case class MediaInfo(mediaRangeType: String, mediaRangeParams: List[(String,String)], qVal: Option[Double], acceptParams: List[(String,String)])

object AcceptHeaderParsers extends JavaTokenParsers {

  def acceptHeader: Parser[List[MediaInfo]] = repsep(mediaInfo, ",")
  
  def mediaInfo: Parser[MediaInfo] = (mediaRange ~ opt(qParam ~ rep(params))) ^^ {
    case range~None => MediaInfo(range._1,range._2,None,Nil)
    case range~Some(q~ap) => MediaInfo(range._1,range._2,Some(q),ap)
  }

  def qParam: Parser[Double] = ";q=" ~> floatingPointNumber ^^ {
    v => {
      val value = v.toDouble
      if (value < 0) 0.0
      else if (value > 1) 1.0
      else value
    } 
  }

  def mediaRange: Parser[(String, List[(String,String)])] = (mediaType ~ rep(params)) ^^ { case head~tail => (head,tail) }
  
  // this is a little off, but im lazy for now
  def params: Parser[(String,String)] = (";" ~> """[^\sq=]+""".r) ~ ("=" ~> """[^,;]+""".r) ^^ { case head~tail => (head,tail) }
  
  def mediaType: Parser[String] = """\*/\*""".r | subTypeAll | fullType
  
  def fullType = ((desc <~ "/".r) ~ desc) ^^ { case s~st => s + "/" + st }
  
  def subTypeAll = (desc ~ """/\*""".r) ^^ { case head~tail => head+tail }
  
  def desc: Parser[String] = """[\w\.-]*""".r


}

gistfile1.scala

scala> parseAll(acceptHeader, "text/html")
List(MediaInfo(text/html,List(),None,List()))

scala> parseAll(acceptHeader, "text/html;q=0.2;a=b,text/plain")
List(MediaInfo(text/html,List(),Some(0.2),List((a,b))), MediaInfo(text/plain,List(),None,List()))

scala> parseAll(acceptHeader, "text/html;version=1;q=0.2;a=b,text/plain;a=0.5")
List(MediaInfo(text/html,List((version,1)),Some(0.2),List((a,b))), MediaInfo(text/plain,List((a,0.5)),None,List()))

scala> parseAll(acceptHeader, "text/html;version=1;q=0.2;a=b,text/plain;q=0.5")
List(MediaInfo(text/html,List((version,1)),Some(0.2),List((a,b))), MediaInfo(text/plain,List(),Some(0.5),List()))