trim-phrase-to-length.scala

object Trimmed {
  val Stop = ".*\\b(a|an|the|to|of|as|in|with|for)$".r
  val Fine = "(.+\\.)$".r
  val Punc = "(.+)[,;:]$".r
}

class Trimmed(size: Int) {
  def unapply(s: String) = s.replaceAll("\\s+", " ") match {
    case res if res.size <= size => Some(res)
    case res => res.split(' ').inits.map(_ mkString " ").flatMap {
      case Trimmed.Stop(_)      => None
      case Trimmed.Fine(prefix) => Some(prefix)
      case Trimmed.Punc(prefix) => Some(prefix + "...")
      case prefix               => Some(prefix + "...")
    }.dropWhile(_.size > size).toList.headOption
  }
}

object Tweetable extends Trimmed(140)

val Tweetable(phrase1) = """Egypt’s military on Wednesday ousted Mohamed Morsi,
  the nation’s first freely elected president, suspending the Constitution,
  installing an interim government and insisting it was responding to the
  millions of Egyptians who had opposed the Islamist agenda of Mr. Morsi and
  his allies in the Muslim Brotherhood.
"""

val Tweetable(phrase2) = """Leslie James Pickering noticed something odd in his
  mail last September: A handwritten card, apparently delivered by mistake,
  with instructions for postal workers to pay special attention to the letters
  and packages sent to his home.
"""

Note that the behavior is slightly different from Joe Wicentowski's XQuery version, which may produce results that are longer than the target length after the ellipsis is added. Here the result is guaranteed not to be longer than the target length.