Regex pattern matching with a variable number of matching capturing groups - regex

Question

A text file should be parsed line by line, using Scala pattern matching and regular expressions. If a line starts with "names:\t" the subsequent tab-separated names should be provided as a Seq[String] (or something similar).
Here a non-working code example:
val Names = "^names:(?:\t([a-zA-Z0-9_]+))+$".r
"names:\taaa\tbbb\tccc" match {
case Names(names # _*) => println(names)
// […] other cases
case _ => println("no match")
}
Output: List(ccc)
Wanted output: List(aaa, bbb, ccc)
The following code works as desired…
object NamesObject {
private val NamesLine = "^names:\t([a-zA-Z0-9_]+(?:\t[a-zA-Z0-9_]+)*)$".r
def unapplySeq(s: String): Option[Seq[String]] = s match {
case NamesLine(nameString) => Some(nameString.split("\t"))
case _ => None
}
}
"names:\taaa\tbbb\tccc" match {
case NamesObject(names # _*) => println(names)
// […] other cases
case _ => println("no match")
}
Output (as wanted): WrappedArray(aaa, bbb, ccc)
I would like to know: Is this is possible in a simpler manner without creating an object, just like in the first but non-working code example?

Answer 1

Use your working regex.(\w is a predefined character class for[a-zA-Z0-9_])
val Names = """names:\t(\w+(?:\t\w+)*)""".r
"names:\taaa\tbbb\tccc" match {
case Names(names) => println(names.split("\t") toSeq)
case _ => println("no match")
}
With first, second & tail bindings,
val Names = """names:\t(\w+)?\t?(\w+)?\t?((?:\w+?\t?)*)""".r
"names:\taaa\tbbb\tccc\tddd" match {
case Names(first, second, tail) =>
println(first + ", " + second + ", " + (tail.split("\t") toSeq));
case _ => println("no match")
}

Answer 2

As Randall Schulz said, it seems not to be possible just using regular expressions. Therefore the short answer to my question would be no.
My current solution is the following: I use the this class…
class SeparatedLinePattern(Pattern: Regex, separator: String = "\t") {
def unapplySeq(s: String): Option[Seq[String]] = s match {
case Pattern(nameString) => Some(nameString.split(separator).toSeq)
case _ => None
}
}
…to create the patterns:
val Names = new SeparatedLinePattern("""names:\t([A-Za-z]+(?:\t[A-Za-z]+)*)""".r)
val Ints = new SeparatedLinePattern("""ints:\t(\d+(?:\t\d+)*)""".r)
val ValuesWithID = new SeparatedLinePattern("""id-value:\t(\d+\t\w+(?:\t\d+\t\w+)*)""".r)
Here an example how they can be used in a quite flexible manner:
val testStrings = List("names:\taaa", "names:\tbbb\tccc", "names:\tddd\teee\tfff\tggg\thhh",
"ints:\t123", "ints:\t456\t789", "ints:\t100\t200\t300",
"id-value:\t42\tbaz", "id-value:\t2\tfoo\t5\tbar\t23\tbla")
for (s <- testStrings) s match {
case Names(name) => println(s"The name is '$name'")
case Names(a, b) => println(s"The two names are '$a' and '$b'")
case Names(names # _*) => println("Many names: " + names.mkString(", "))
case Ints(a) => println(s"Just $a")
case Ints(a, b) => println(s"$a + $b == ${a.toInt + b.toInt}")
case Ints(nums # _*) => println("Sum is " + (nums map (_.toInt)).sum)
case ValuesWithID(id, value) => println(s"ID of '$value' is $id")
case ValuesWithID(values # _*) => println("As map: " + (values.grouped(2) map (x => x(0).toInt -> x(1))).toMap)
case _ => println("No match")
}
Output:
The name is 'aaa'
The two names are 'bbb' and 'ccc'
Many names: ddd, eee, fff, ggg, hhh
Just 123
456 + 789 == 1245
Sum is 600
ID of 'baz' is 42
As map: Map(2 -> foo, 5 -> bar, 23 -> bla)