Parsing case statements in scala - regex

Question

Parsing case statements in scala
CASE WHEN col1 <> 0 AND col2 <> 0 THEN 'COL1 & COL2 IS NOT ZERO' ELSE 'COL1 & COL2 IS ZERO'
challenge here is to give all the scenarios where case statement can come for e.g. it can come inside a function. Also case statements/functions etc. can come inside another case statements which has to be handled.

Answer 1

This problem can be solved with scala parser combinator
first define the classes needed to map experssions
sealed trait Exp {
def asStr: String
override def toString: String = asStr
}
case class OperationExp(a: Exp, op: String, b: Exp, c: Option[String]) extends Exp { override def asStr = s"$a $op $b ${c.getOrElse("")}" }
case class CaseConditions(conditionValue: List[(String, String)] , elseValue: String, asAlias: Option[Exp]) extends Exp {
override def asStr = "CASE " + conditionValue.map(c => s"WHEN ${c._1} THEN ${c._2}").mkString(" ") + s" ELSE ${elseValue} END ${asAlias.getOrElse("")}"
}
now the solution
case class OperationExp(a: Exp, op: String, b: Exp, c: Option[String]) extends Exp { override def asStr = s"$a $op $b ${c.getOrElse("")}" }
case class CaseConditions(conditionValue: List[(String, String)] , elseValue: String, asAlias: Option[Exp]) extends Exp {
override def asStr = "CASE " + conditionValue.map(c => s"WHEN ${c._1} THEN ${c._2}").mkString(" ") + s" ELSE ${elseValue} END ${asAlias.getOrElse("")}"
}
val identifiers: Parser[String] = "[a-zA-Z0-9_~\\|,'\\-\\+:.()]+".r
val operatorTokens: Parser[String] = "[<>=!]+".r | ("IS NOT" | "IN" | "IS")
val conditionJoiner: Parser[String] = ( "AND" | "OR" )
val excludeKeywords = List("CASE","WHEN", "THEN", "ELSE", "END")
val identifierWithoutCaseKw: Parser[Exp] = Parser(input =>
identifiers(input).filterWithError(
!excludeKeywords.contains(_),
reservedWord => s"$reservedWord encountered",
input
)
) ^^ StrExp
val anyStrExp: Parser[Exp] = "[^()]*".r ^^ StrExp
val funcIdentifier: Parser[Exp] = name ~ ("(" ~> (caseConditionExpresionParser | funcIdentifier | anyStrExp) <~ ")") ^^ {case func ~ param => FunCallExp(func, Seq(param))}
val identifierOrFunctions = funcIdentifier | identifierWithoutCaseKw
val conditionParser: Parser[String] =
identifierOrFunctions ~ operatorTokens ~ identifierOrFunctions ~ opt(conditionJoiner) ^^ {
case a ~ op ~ b ~ c => s"$a $op $b ${c.getOrElse("")}"
}
def caseConditionExpresionParser: Parser[CaseConditions] = "CASE" ~ rep1("WHEN" ~ rep(conditionParser) ~ "THEN" ~ rep(identifierWithoutCaseKw)) ~ "ELSE" ~ rep(identifierWithoutCaseKw) ~ "END" ~ opt("AS" ~> identifierWithoutCaseKw)^^ {
case "CASE" ~ conditionValuePair ~ "ELSE" ~ falseValue ~ "END" ~ asName =>
CaseConditions(
conditionValuePair.map(cv => (
cv._1._1._2.mkString(" "),
parsePipes(cv._2.mkString(" ")).isRight match {
case true => parsePipes(cv._2.mkString(" ")).right.get
case _ => cv._2.mkString(" ")
}
)),
parsePipes(falseValue.mkString("")).isRight match {
case true => parsePipes(falseValue.mkString(" ")).right.get
case _ => falseValue.mkString("")
}, asName)
}
//this parser can be used to get the results
val caseExpression = caseConditionExpresionParser | funcIdentifier
def parsePipes(input: String): Either[Seq[ParsingError], String] = {
parse(caseExpression, input) match {
case Success(parsed, _) => Right(parsed.asStr)
case Failure(msg, next) => Left(Seq(ParsingError(s"Failed to parse $pipedStr: $msg, next: ${next.source}.")))
case Error(msg, next) => Left(Seq(ParsingError(s"Error in $pipedStr parse: $msg, next: ${next.source}.")))
}
}