How to check if a string is actually a number without attempting to convert it first, in Scala

gistfile1.scala

//
// Compile and run:
//  scalac isnumber.scala
//  scala IsNumber
//

// this is the class that provides the isNumber method when called on java.lang.String
class ExtendedString(s:String) {
	def isNumber: Boolean = s.matches("[+-]?\\d+.?\\d+")	
}

// and this is the companion object that provides the implicit conversion
object ExtendedString {
	implicit def String2ExtendedString(s:String) = new ExtendedString(s)
}

import ExtendedString._
object IsNumber extends App {
	println("Implicit 1: " + "123".isNumber)
	println("Implicit 2:" + "123a".isNumber)
	println("Implicit 3:" + "123.55".isNumber)

	val l = List("1", "3", "awdf", "123")
	println("Elements in list: "  + l.forall(_.isNumber))
}

According to this "2" is not a number and "2+3", "2*3" etc. are.

I used this.
def isNumeric(str:String): Boolean = str.matches("[-+]?\\d+(\\.\\d+)?")