Question

Let’s say I want to parse a string with various opening and closing brackets (I used parentheses in the title because I believe it is more common — the question is the same nevertheless) so that I get all the higher levels separated in a list.

Given:

[hello:=[notting],[hill]][3.4(4.56676|5.67787)][the[hill[is[high]]not]]

I want:

List("[hello:=[notting],[hill]]", "[3.4(4.56676|5.67787)]", "[the[hill[is[high]]not]]")

The way I am doing this is by counting the opening and closing brackets and adding to the list whenever I get my counter to 0. However, I have an ugly imperative code. You may assume that the original string is well formed.

My question is: what would be a nice functional approach to this problem?

Notes: I have thought of using the for…yield construct but given the use of the counters I cannot get a simple conditional (I must have conditionals just for updating the counters as well) and I do not know how I could use this construct in this case.

Answer 1

Quick solution using Scala parser combinator library:

import util.parsing.combinator.RegexParsers

object Parser extends RegexParsers {
  lazy val t = "[^\\[\\]\\(\\)]+".r

  def paren: Parser[String] =
    ("(" ~ rep1(t | paren) ~ ")" |
     "[" ~ rep1(t | paren) ~ "]") ^^ {
      case o ~ l ~ c => (o :: l ::: c :: Nil) mkString ""
    }

  def all = rep(paren)

  def apply(s: String) = parseAll(all, s)
}

Checking it in REPL:

scala> Parser("[hello:=[notting],[hill]][3.4(4.56676|5.67787)][the[hill[is[high]]not]]")
res0: Parser.ParseResult[List[String]] = [1.72] parsed: List([hello:=[notting],[hill]], [3.4(4.56676|5.67787)], [the[hill[is[high]]not]])