function validateRegon (regon) {

  //REGON is a 9 or 14 digit number. Last digit is control digit from equation:
  // [ sum from 1 to (9 or 14) (x[i]*w[i]) ] mod 11; where x[i] is pointed NIP digit and w[i] is pointed digit
  //from [8 9 2 3 4 5 6 7] for 9 and [2 4 8 5 0 9 7 3 6 1 2 4 8] for 14 digits.

  var n = regon.length;
  var w;
  var cd = 0; // Control digit (last digit)
  var isOnlyDigit = /^\d+$/.test(regon);

  if ( n !==9 && n !== 14 && !isOnlyDigit) {
    console.log("Error");
    return false;
  }

  if ( n === 9) {
    w = [8, 9, 2, 3, 4, 5, 6, 7];
  } else {
    w = [2, 4, 8, 5, 0, 9, 7, 3, 6, 1, 2, 4, 8];
  }

  for (var i = 0; i<n-1; i++) {
    cd += w[i]*parseInt(regon.charAt(i));
  }

  cd %= 11;

  if ( cd === 10 ) {
    cd = 0;
  }

  if ( cd !== parseInt(regon.charAt(n-1)) ) {
    console.log("Not valid");
    return false;
  } else {
    console.log("Valid!");
  }



}

// validateRegon("999999999"); // 9 digit
// validateRegon("99999999999999"); // 14 digit
// validateRegon("793769692"); // valid 9 digits Regon
// validateRegon("63867185528390"); //valid 14 digits Regon