Cannot create constructor with logic in Scala program?

Try to introduce a helper method

import scala.util.matching.Regex

def gcd(i: Int, i1: Int): Int = BigInt(i).gcd(BigInt(i1)).toInt

class Rational(n: Int, d: Int) {
  require(d != 0)
  private val g = gcd(n.abs, d.abs)
  val numer = n / g
  val denom = d / g

  def this(n: Int) = this(n, 1)

  def this(s: String) = {
    this(Rational.transformStr(s)._1, Rational.transformStr(s)._2)
  }
}

object Rational {
  // helper method
  def transformStr(s: String): (Int, Int) = {
    val regex: Regex = "^([+-]?(\\d+|\\d*\\.?\\d+)|\\d*\\/?\\d+)$".r
    if (!regex.matches(s)) throw new NumberFormatException()
    val input: Array[String] = s.split("\\.|\\/")
    val num: Int = input(0).toInt
    if (input.length equals 1)
      (num, 1)
    else
      (num, input(1).toInt)
  }
}

or better, factory methods (because constructors have many limitations)

class Rational(n: Int, d: Int) {
  require(d != 0)
  private val g = gcd(n.abs, d.abs)
  val numer = n / g
  val denom = d / g
}

object Rational {
  // factory methods
  def apply(n: Int) = new Rational(n, 1)

  def apply(s: String): Rational = {
    val regex: Regex = "^([+-]?(\\d+|\\d*\\.?\\d+)|\\d*\\/?\\d+)$".r
    if (!regex.matches(s)) throw new NumberFormatException()
    val input: Array[String] = s.split("\\.|\\/")
    val num: Int = input(0).toInt
    if (input.length equals 1)
      new Rational(num, 1)
    else
      new Rational(num, input(1).toInt)
  }
}

Executing code in overloaded constructor prior to calling this()

By the way, you can also use default values

class Rational(n: Int, d: Int = 1 /*default*/ ) {
  // ...
}

object Rational {
  def apply(s: String): Rational = ???
}