Accessing Scala Parser regular expression match data

Question

I wondering if it's possible to get the MatchData generated from the matching regular expression in the grammar below.

object DateParser extends JavaTokenParsers {

    ....

    val dateLiteral = """(\d{4}[-/])?(\d\d[-/])?(\d\d)""".r ^^ {
        ... get MatchData
    }
}

One option of course is to perform the match again inside the block, but since the RegexParser has already performed the match I'm hoping that it passes the MatchData to the block, or stores it?

Answer 1

When a Regex is used in a RegexParsers instance, the implicit def regex(Regex): Parser[String] in RegexParsers is used to appoly that Regex to the input. The Match instance yielded upon successful application of the RE at the current input is used to construct a Success in the regex() method, but only its "end" value is used, so any captured sub-matches are discarded by the time that method returns.

As it stands (in the 2.7 source I looked at), you're out of luck, I believe.

Answer 2

No, you can't do this. If you look at the definition of the Parser used when you convert a regex to a Parser, it throws away all context and just returns the full matched string:

http://lampsvn.epfl.ch/trac/scala/browser/scala/tags/R%5F2%5F7%5F7%5Ffinal/src/library/scala/util/parsing/combinator/RegexParsers.scala?view=markup#L55

You have a couple of other options, though:

• break up your parser into several smaller parsers (for the tokens you actually want to extract)
• define a custom parser that extracts the values you want and returns a domain object instead of a string

The first would look like

val separator = "-" | "/"
  val year = ("""\d{4}"""r) <~ separator
  val month = ("""\d\d"""r) <~ separator
  val day = """\d\d"""r

  val date = ((year?) ~ (month?) ~ day) map {
    case year ~ month ~ day =>
      (year.getOrElse("2009"), month.getOrElse("11"), day)
  }

The <~ means "require these two tokens together, but only give me the result of the first one.

The ~ means "require these two tokens together and tie them together in a pattern-matchable ~ object.

The ? means that the parser is optional and will return an Option.

The .getOrElse bit provides a default value for when the parser didn't define a value.

Answer 3

Here is the implicit definition that converts your Regex into a Parser:

  /** A parser that matches a regex string */
  implicit def regex(r: Regex): Parser[String] = new Parser[String] {
    def apply(in: Input) = {
      val source = in.source
      val offset = in.offset
      val start = handleWhiteSpace(source, offset)
      (r findPrefixMatchOf (source.subSequence(start, source.length))) match {
        case Some(matched) =>
          Success(source.subSequence(start, start + matched.end).toString, 
                  in.drop(start + matched.end - offset))
        case None =>
          Failure("string matching regex `"+r+"' expected but `"+in.first+"' found", in.drop(start - offset))
      }
    }
  }

Just adapt it:

object X extends RegexParsers {
  /** A parser that matches a regex string and returns the Match */
  def regexMatch(r: Regex): Parser[Regex.Match] = new Parser[Regex.Match] {
    def apply(in: Input) = {
      val source = in.source
      val offset = in.offset
      val start = handleWhiteSpace(source, offset)
      (r findPrefixMatchOf (source.subSequence(start, source.length))) match {
        case Some(matched) =>
          Success(matched,
                  in.drop(start + matched.end - offset))
        case None =>
          Failure("string matching regex `"+r+"' expected but `"+in.first+"' found", in.drop(start - offset))
      }
    }
  }
  val t = regexMatch("""(\d\d)/(\d\d)/(\d\d\d\d)""".r) ^^ { case m => (m.group(1), m.group(2), m.group(3)) }
}

Example:

scala> X.parseAll(X.t, "23/03/1971")
res8: X.ParseResult[(String, String, String)] = [1.11] parsed: (23,03,1971)