Matching braces code doesn't terminate - scala

Question

I have this code below to check a string. We want to verify that it starts with '{' and ends with '}' and that it contains sequences of non-"{}" characters and strings that also have this property.
import util.parsing.combinator._
class Comp extends RegexParsers with PackratParsers {
lazy val bracefree: PackratParser[String] = """[^{}]*""".r ^^ {
case a => a
}
lazy val matching: PackratParser[String] = (
"{" ~ rep(bracefree | matching) ~ "}") ^^ {
case a ~ b ~ c => a + b.mkString("") + c
}
}
object Brackets extends Comp {
def main(args: Array[String])= {
println(parseAll(matching, "{ foo {hello 3 } {}}").get)
}
}
The desired output for this is to echo { foo {hello 3 } {}}, but it ends up taking a long time before dying from java.lang.OutOfMemoryError: GC overhead limit exceeded. What am I doing wrong and what should I have done instead?

Answer 1

Your regular expression for bracefree string matches even an empty string, so parser produced by rep() succeeds without consuming any input and will loop endlessly.
Use a + quantifier instead of *:
lazy val bracefree: PackratParser[String] = """[^{}]+""".r ^^ {
case a => a
}
Also, by default RegexParsers will skip empty strings and whitespaces. To turn that behavior off, just override method skipWhitespace to always return false. In the end your parser will look like this:
import util.parsing.combinator._
class Comp extends RegexParsers with PackratParsers {
override def skipWhitespace = false
lazy val bracefree: PackratParser[String] = """[^{}]+""".r ^^ {
case a => a
}
lazy val matching: PackratParser[String] = (
"{" ~ rep(bracefree | matching) ~ "}") ^^ {
case a ~ b ~ c => a + b.mkString("") + c
}
}
object Brackets extends Comp {
def main(args: Array[String])= {
println(parseAll(matching, "{ foo {hello 3 } {}}").get)
// prints: { foo {hello 3 } {}}
}
}