Scala: Matching optional Regular Expression groups

regex, scala, scala-2.8

Solution

The optional group will be null if it is not matched so you need to include "null" in the pattern match:

import scala.xml._

val StatementPattern = """([\w\.]+)\s*:\s*([+-])?(\d+)""".r

def buildProperty(input: String): Node = input match {
    case StatementPattern(name, null, value) => <propertyWithoutSign />
    case StatementPattern(name, sign, value) => <propertyWithSign />
}

val withSign = "property.name: +10"
val withoutSign = "property.name: 10"

buildProperty(withSign)        // <propertyWithSign></propertyWithSign>
buildProperty(withoutSign)     // <propertyWithSign></propertyWithSign>

Problem

Original source