regex differentiating between ISBN-10 and ISBN-13

Question

I have an If-else statement which checks a string to see whether there is an ISBN-10 or ISBN-13 (book ID).

The problem I am facing is with the ISBN-10 check which o

Answer 1

Use ^ and $ to match beginning and end of string. By using the string delimiters, the order in which you test the 10 or the 13-digit codes will not matter.

10 digits

/^ISBN:(\d{9}(?:\d|X))$/

---

13 digits

/^ISBN:(\d{12}(?:\d|X))$/

---

Note: According to http://en.wikipedia.org/wiki/International_Standard_Book_Number, it appears as though ISBNs can have a - in them as well. But based on the $str you're using, it looks like you've removed the hyphens before checking for 10 or 13 digits.

Additional note: Because the last digit of the ISBN is used as a sort of checksum for the prior digits, regular expressions alone cannot validate that the ISBN is a valid one. It can only check for 10 or 13-digit formats.

---

$isbns = array(
  'ISBN:1234567890',       // 10-digit
  'ISBN:123456789X',       // 10-digit ending in X
  'ISBN:1234567890123',    // 13-digit
  'ISBN:123456789012X',    // 13-digit ending in X
  'ISBN:1234'              // invalid
);

function get_isbn($str) {
   if (preg_match('/^ISBN:(\d{9}(?:\d|X))$/', $str, $matches)) {
      echo "found 10-digit ISBN\n";
      return $matches[1];
   }
   elseif (preg_match('/^ISBN:(\d{12}(?:\d|X))$/', $str, $matches)) {
      echo "found 13-digit ISBN\n";
      return $matches[1];
   }
   else {
      echo "invalid ISBN\n";
      return null;
   }
}

foreach ($isbns as $str) {
   $isbn = get_isbn($str);
   echo $isbn."\n\n";
}

Output

found 10-digit ISBN
1234567890

found 10-digit ISBN
123456789X

found 13-digit ISBN
1234567890123

found 13-digit ISBN
123456789012X

invalid ISBN