Scala: Matching optional Regular Expression groups

Question

I'm trying to match on an option group in Scala 2.8 (beta 1) with the following code:

import scala.xml._

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

def buildProperty(input: String): Node = input match {
    case StatementPattern(name, 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>

But this is not working. What is the correct way to match optional regex groups?

Answer 1

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>

Answer 2

I don't see any problem with your regex. Although you need not escape the . in the char class.

EDIT:

You can try something like:

([\w.]+)\s*:\s*((?:+|-)?\d+)

To capture the name and value where the value can have an optional sign.