How to match a string on a prefix and get the rest?

Starting Scala 2.13, it's now possible to pattern match a String by unapplying a string interpolator:

"!!!hello" match {
  case s"!!!$rest" => rest
  case _           => "oups"
}
// "hello"

If it's the sort of thing you do often, it's probably worth creating an extractor

object BangBangBangString{ 
   def unapply(str:String):Option[String]= {
       str match {
          case s if s.startsWith("!!!") => Some(s.stripPrefix("!!!"))
          case _ => None
       }
   }
}

Then you can use the extractor as follows

str match{
   case BangBangBangString(rest) => println(rest)
   case _ => println("Doesn't start with !!!")
}

or even

for(BangBangBangString(rest)<-myStringList){
   println("rest")
}

val r = """^!!!(.*)""".r
val r(suffix) = "!!!rest of string"

So suffix will be populated with rest of string, or a scala.MatchError gets thrown.

A different variant would be:

val r = """^(!!!){0,1}(.*)""".r
val r(prefix,suffix) = ...

And prefix will either match the !!! or be null. e.g.

(prefix, suffix) match {
   case(null, s) => "No prefix"
   case _ => "Prefix"
}

The above is a little more complex than you might need, but it's worth looking at the power of Scala's regexp integration.