PLSQL - Trigger for validating ISBN numbers

CREATE OR REPLACE TRIGGER check_isbn
  BEFORE INSERT ON buch
  FOR EACH ROW 
DECLARE
  -- isbn is nine chars long and exists of numbers only
  new_isbn NUMBER;
  new_check_digit CHAR;
  char_at NUMBER;
  summe NUMBER;
  i NUMBER;
BEGIN
  new_isbn := TO_CHAR(:new.isbn);	
  new_check_digit := TO_CHAR(:new.ISBN_Pruefziffer);
  summe := 0;
  
  -- Replace '-' to ''
  -- replace(new_isbn, '-');
  
  -- We should have 9 numbers in new_isbn
  IF NOT LENGTH(new_isbn) = 9 THEN
    RAISE_APPLICATION_ERROR(-20012, 'The ISBN number supplied is invalid!');
  END IF;
   
  -- Loop 9 times from 10 to 2
  FOR i IN REVERSE 2..10
  LOOP
    -- Get position of current number
    char_at := (11 - i);
    
    summe := summe + (i * TO_NUMBER(SUBSTR(new_isbn, char_at, 1)));
  END LOOP;

  -- Check for 'X' as last digit
  -- Add the value of 'new_check_digit' to 'sum'
  IF new_check_digit = 'X' THEN
    summe := summe + 10;
  ELSE
    summe := summe + TO_NUMBER(new_check_digit);
  END IF;
   
  IF NOT mod(summe, 11) = 0 THEN
    RAISE_APPLICATION_ERROR(-20012, 'The ISBN number supplied is invalid!');
  END IF;
END;
/