Exercise 12-07:
Write a program that validates user data entry. It will validate ISBNs (International Standard Book Numbers), which are used to identify books worldwide.
The program must accept ISBNs in the following format: ISBN 999-9-99-999999-9, where each 9 represents a digit.
The ISBN letters are optional and be case-insensitive. The space between N and the first number is mandatory if the ISBN is present. Require the dashes as in the example, verifying the correct number of digits.
Valid values:
ISBN 123-3-12-123456-1
Isbn 123-3-12-123456-1
123-3-12-123456-1
Invalid values:
ISBN123-3-12-123456-1
ISBN 1233-12123456-1
Answer:
##############################################################################
# Python From Scratch
# Author: Nilo Ney Coutinho Menezes
# Editora Novatec (c) 2010-2025 - LogiKraft 2025
# Site: https://pythonfromscratch.com
# ISBN: 978-85-7522-949-1 (Paperback), 978-85-7522-950-7 (hardcover), 978-85-7522-951-4 (ebook)
#
# File: chapter 12/exercise-12-07.py.py
##############################################################################
import re
def validate_isbn(isbn):
# Regex pattern to validate the ISBN format
# ^ - start of string
# (?:ISBN\s+)? - optional "ISBN" followed by at least one space
# \d{3}-\d{1}-\d{2}-\d{6}-\d{1} - specific format with hyphens
# $ - end of string
# flags=re.IGNORECASE - ignore case for "ISBN"
pattern = r"^(?:ISBN\s+)?\d{3}-\d{1}-\d{2}-\d{6}-\d{1}$"
return re.match(pattern, isbn, flags=re.IGNORECASE)
print("Enter ISBNs for validation (or 'exit' to quit)")
print("Expected format: ISBN 999-9-99-999999-9\n")
while True:
isbn_input = input("Enter an ISBN: ").strip()
# Check if the user wants to exit
if isbn_input.lower() in ["sair", "exit", "quit", "q"]:
print("Program terminated.")
break
# Validate the ISBN
if validate_isbn(isbn_input):
print("✓ VALID ISBN\n")
else:
print("✗ INVALID ISBN")
print(" Expected format: ISBN 999-9-99-999999-9")
print(" Example: ISBN 123-3-12-123456-1\n")