Reversing a Process

import scala.collection.Map
import scala.util.Try

object Decode {
  val pattern = "(\\d+)(\\w+)".r

  def encoding(num: Int): Map[Char, Char] =
    ('a' to 'z')
        .map(c => (((c - 'a') * num % 26 + 'a').toChar, c))
        .toMap

  def decode(r: String): String = r match {
    case pattern(num, s) =>
      val enc = encoding(num.toInt)
      if (enc.size == 26) s.map(enc)
      else "Impossible to decode"

    case _ => "Invalid pattern"
  }
}