_ * usage is scala pattern matching

Question

I am bit confused with _ * usage in the below example.It is behaving differently and I am completely lost in figuring this out .

val p1="scala".r
val p2="([0-9]+[.]?[0-9]*|[.][0-9]+)$".r

"scala" match {case p1(_) => true case _  => false } // return false


"scala" match {case p1(_ *) => true case _  => false }  // return true

"9" match {case p2(_) => true case _ => false} // return true

Thanks in Advance.

Answer 1

The match needs at least one capture group in the pattern because you are using case(_) . This is because "match-case" uses unapplySeq(target: Any): Option[List[String]] , which returns the capture group values, and you require 1 captured value to be present with _ . With _ * , you ignore this requirement. See Scala regex reference :

To check only whether the Regex matches, ignoring any groups, use a sequence wildcard :

"2004-01-20" match { case date(_*) => "It's a date!" }

Your p2 contains a capturing group, hence the last line returns true.

Your p1 does not contain a capturing group, hence the first match fails. The second does not since you disabled this requirement with * .

As an alternative, you can use case p1() with your first line (the _ representing the obligatory first capture group is removed here), and it will work, too:

"scala" match {case p1() => true case _  => false }) // return true

See this IDEONE demo