ex4 - https://medium.com/@lu4nm3/use-smart-constructors-to-create-safe-apis-c3cbd9ac205d

ex4_smart.scala

object EmailAddress {

  // Regex that captures the "official" format standard (RFC 5322) that email addresses must adhere to
  private val EmailRegex = """(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21\x23-\x5b\x5d-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])*")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?|\[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[a-z0-9]:(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21-\x5a\x53-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])+)\])""".r

  private def isValidAddress(emailAddress: String): Boolean = {
    EmailRegex.findAllMatchIn(emailAddress).length == 1
  }

  def apply(emailAddress: String): EmailAddress = {
    require(isValidAddress(emailAddress), "Invalid email address")
    new EmailAddress(emailAddress)
  }
}