Check if regex pattern matching failed

Question

I am working on a function that extracts a number at the beginning of the string. Given a string it should return a case class Success (containing the extracted number in Int and the rest of the string) in case it found a number at the beginning and a case Object Failure otherwise.

For that, I'm using regex pattern matching to match the number and the rest of the string into two separate variable. I then convert the number to Int and return.

The problem: when the string provided does not contain a number at the beginning, the pattern matching fail but I don't know how to detect that it fails, and when to return my Failure.

Any ideas ?

Here is my code

sealed trait Result
case class Success(expr: String, result: Int) extends Result
case object Failure extends Result
def extractNumber(str: String) : Result = {
 val pattern = "([0-9]+)(.*)".r
 val pattern(count, rest) = str
 if (pattern match failed) //How can I do that
 return Failure
 val number = count.toInt
 return Success(rest, number)
}
extractNumber("100banana") match {
 case Success(rest, nb) => println("sucess" + nb + ";" + rest)
 case Failure => println("failure")
}

Answer 1

You can pattern match on the string safely and extractor the required information if pattern matches.

In the below pattern can be used as an extractor. Read about Scala extractor pattern.

def extractNumber(str: String) : Result = {
 val pattern = "([0-9]+)(.*)".r
 str match {
 case pattern(a, b) => Success(b, a.toInt)
 case _ => Failure
 }
}

Lets check if it works on Scala repl

scala> :paste
// Entering paste mode (ctrl-D to finish)
sealed trait Result
case class Success(expr: String, result: Int) extends Result
case object Failure extends Result
def extractNumber(str: String) : Result = {
 val pattern = "([0-9]+)(.*)".r
 str match {
 case pattern(a, b) => Success(b, a.toInt)
 case _ => Failure
 }
}
// Exiting paste mode, now interpreting.
defined trait Result
defined class Success
defined object Failure
extractNumber: (str: String)Result
scala> extractNumber("123192731qwlejqwlkje")
res0: Result = Success(qwlejqwlkje,123192731)

Looks like it works

Lets check what happens in case of failure.

scala> extractNumber("qwlejqwlkje")
res1: Result = Failure

Answer 2

Instead of using deconstruction:

val pattern(count, rest) = str

Try using one of the Regex API methods:

• findFirstIn returns an option -> if None, then this is "failure"
• findAllIn returns a MatchIterator that can tell info about the failure (for instance, hasNext or whatever)

An other way is to perform just as you do, inside a try ... catch and catch the MatchError.