How to pattern match using regular expression in Scala?

I would like to be able to find a match between the first letter of a word, and one of the letters in a group such as "ABC". In pseudocode, this might look something like:

case Process(word) =>
   word.firstLetter match {
      case([a-c][A-C]) =>
      case _ =>
   }
}

But how do I grab the first letter in Scala instead of Java? How do I express the regular expression properly? Is it possible to do this within a case class?

Solution 1:

You can do this because regular expressions define extractors but you need to define the regex pattern first. I don't have access to a Scala REPL to test this but something like this should work.

val Pattern = "([a-cA-C])".r
word.firstLetter match {
   case Pattern(c) => c bound to capture group here
   case _ =>
}

Solution 2:

Since version 2.10, one can use Scala's string interpolation feature:

implicit class RegexOps(sc: StringContext) {
  def r = new util.matching.Regex(sc.parts.mkString, sc.parts.tail.map(_ => "x"): _*)
}

scala> "123" match { case r"\d+" => true case _ => false }
res34: Boolean = true

Even better one can bind regular expression groups:

scala> "123" match { case r"(\d+)$d" => d.toInt case _ => 0 }
res36: Int = 123

scala> "10+15" match { case r"(\d\d)${first}\+(\d\d)${second}" => first.toInt+second.toInt case _ => 0 }
res38: Int = 25

It is also possible to set more detailed binding mechanisms:

scala> object Doubler { def unapply(s: String) = Some(s.toInt*2) }
defined module Doubler

scala> "10" match { case r"(\d\d)${Doubler(d)}" => d case _ => 0 }
res40: Int = 20

scala> object isPositive { def unapply(s: String) = s.toInt >= 0 }
defined module isPositive

scala> "10" match { case r"(\d\d)${d @ isPositive()}" => d.toInt case _ => 0 }
res56: Int = 10

An impressive example on what's possible with Dynamic is shown in the blog post Introduction to Type Dynamic:

object T {

  class RegexpExtractor(params: List[String]) {
    def unapplySeq(str: String) =
      params.headOption flatMap (_.r unapplySeq str)
  }

  class StartsWithExtractor(params: List[String]) {
    def unapply(str: String) =
      params.headOption filter (str startsWith _) map (_ => str)
  }

  class MapExtractor(keys: List[String]) {
    def unapplySeq[T](map: Map[String, T]) =
      Some(keys.map(map get _))
  }

  import scala.language.dynamics

  class ExtractorParams(params: List[String]) extends Dynamic {
    val Map = new MapExtractor(params)
    val StartsWith = new StartsWithExtractor(params)
    val Regexp = new RegexpExtractor(params)

    def selectDynamic(name: String) =
      new ExtractorParams(params :+ name)
  }

  object p extends ExtractorParams(Nil)

  Map("firstName" -> "John", "lastName" -> "Doe") match {
    case p.firstName.lastName.Map(
          Some(p.Jo.StartsWith(fn)),
          Some(p.`.*(\\w)$`.Regexp(lastChar))) =>
      println(s"Match! $fn ...$lastChar")
    case _ => println("nope")
  }
}