Scala Regex小于等于运算符（

I am trying to parse an expression with (<, <=, >=, >). All but <= works just fine. Can someone help what could be the issue. Code:

我试图用（<，<=，> =，>）解析表达式。除<=之外的所有工作都很好。有人可以帮助解决问题。码：

object MyTestParser extends RegexParsers {
  override def skipWhitespace = true

  private val expression: Parser[String] = """[a-zA-Z0-9\.]+""".r

  val operation: Parser[Try[Boolean]] =
    expression ~ ("<" | "<=" | ">=" | ">") ~ expression ^^ {
      case v1 ~ op ~ v2 => for {
        a <- Try(v1.toDouble)
        b <- Try(v2.toDouble)
      } yield op match {
        case "<" => a < b
        case "<=" => a <= b
        case ">" => a > b
        case ">=" => a >= b
      }
  }
}

Test:

测试：

"MyTestParser" should {
    "successfully parse <= condition" in {
      val parser = MyTestParser.parseAll(MyTestParser.operation, "10 <= 20")
      val result = parser match {
        case MyTestParser.Success(s, _) => s.get
        case MyTestParser.Failure(e, _) =>
          println(s"Parsing failed with error: $e")
          false
        case MyTestParser.Error(e, _) =>
          println(s"Parsing error: $e")
          false
      }
      result === true
    }

    "successfully parse >= condition" in {
      val result = MyTestParser.parseAll(MyTestParser.operation, "50 >= 20").get
      result === scala.util.Success(true)
    }
  }

Error for <= condition:

<=条件的错误：

Parsing failed with error: string matching regex `[a-zA-Z0-9\.]+' expected but `=' found

3 个解决方案

#1

You need to change the order of the alternatives so that the longest options could be checked first.

您需要更改备选项的顺序，以便可以首先检查最长的选项。

expression ~ ( "<=" | ">=" | ">" | "<") ~ expression ^^ {

If the shortest alternative matches first, others are not considered at all.

如果最短的替代方案首先匹配，则根本不考虑其他方案。

Also note that a period does not have to be escaped inside a character class, this will do:

另请注意，句点不必在字符类中进行转义，这样做：

"""[a-zA-Z0-9.]+""".r

#2

Your problem is that "<" is matched by <=, so it moves on to trying the expression. If you change the order so that "<=" comes first, that will be matched instead, and you will get the desired result.

您的问题是“<”与<=匹配，因此它继续尝试表达式。如果您更改顺序以便首先出现“<=”，那么将匹配，您将获得所需的结果。

#3

@Prateek: it does not work cause the regex engine works just like a boolean OR. It does not search further if one of the patterns in the or-chain is satisfied at a certain point.

@Prateek：它不起作用，因为正则表达式引擎就像布尔OR一样工作。如果在某个点满足or链中的一个模式，则不会进一步搜索。

So, when use | between patterns, if two or more patterns have substring in common, you have to place the longest first.

所以，当使用|在模式之间，如果两个或多个模式具有共同的子字符串，则必须先放置最长的模式。

As a general rule: order the patterns starting from the longest to the shortest.

作为一般规则：从最长到最短的顺序排列模式。

Change the relevant line like this make it works:

像这样更改相关行使其工作：

 // It works as expected with '>= / >' also before for the same reason
 expression ~ ("<=" | "<" | ">=" | ">") ~ expression ^^ {

Or you want to follow the general rule:

或者你想遵循一般规则：

 expression ~ ("<=" | ">=" | "<" | ">") ~ expression ^^ {

#1