"Collapse" runs of some target item in a sequence.

collapse.scala

def collapse(s: String, target: Char): String = collapse(s.toSeq, target).mkString

def collapse[T](s: Seq[T], target: T): Seq[T] = s match {
  case Seq() => Seq[T]()
  case t0 +: t1 +: tail if t0 == target && t1 == target =>
    collapse(target +: tail, target)
  case t +: tail => t +: collapse(tail, target)
}

/*
// REPL...

scala> collapse("abcdefg___hijk_".toSeq, '_')
res3: Seq[Char] = List(a, b, c, d, e, f, g, _, h, i, j, k, _)

scala> collapse("abcdefg___hijk_", '_')
res4: String = abcdefg_hijk_
*/