Regex not matching

I expect regex "[a-zA-Z]\\d{6}" to match "z999999" but it is not matching as an empty List is mapped :

val lines = List("z999999");                      //> lines  : List[String] = List(z999999)

val regex = """[a-zA-Z]\d{6}""".r                 //> regex  : scala.util.matching.Regex = [a-zA-Z]\d{6}

val fi = lines.map(line => line match { case regex(group) => group case _ => "" })
                                                  //> fi  : List[String] = List("")

Is there a problem with my regex or how I'm using it with Scala ?

Copyright Notice：Content Author:「blue-sky」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/28024438/regex-not-matching