An ISBN is a unique identifier for commercial books and book-like products. The 10-digit ISBN format was published as an international standard, ISO 2108, in 1970. All ISBNs assigned since January 1, 2007 are 13 digits.

ISBN-10 and ISBN-13 numbers are divided into four or five elements, respectively. Three of the elements are of variable length; the remaining one or two elements are of fixed length. All five parts are usually separated with hyphens or spaces. A brief description of each element follows:

All three regex solutions shown earlier are composed of similar parts, so here we’ll focus on the “ISBN-10 or ISBN-13” regex. Its leading ‹^(?:ISBN(?:-1[03])?:?●)?› part has three optional elements, allowing it to match any one of the following seven strings (all except the empty-string option include a space character at the end):

After the leading ‹^(?:ISBN(?:-1[03])?:?●)?› that we just discussed, there is a positive lookahead that enforces one of four options (separated by the ‹|› alternation operator) for the length and character set of the rest of the match, as well as the number of allowed separators (zero or three for ISBN-10s, and zero or four for ISBN-13s). Because there are four alternatives within it, the lookahead is quite long. Here’s the full lookahead: ‹(?=[0-9X]{10}$|(?=(?:[0-9]+[-●]){3})[-●0-9X]{13}$|97[89][0-9]{10}$|(?=(?:[0-9]+[-●]){4})[-●0-9]{17}$)›. Since that’s difficult to analyze on its own, each of the four options within it are shown next. They all end with the ‹$› anchor, which ensures that there cannot be any trailing text that doesn’t fit into one of the patterns:

Two of these options (the ones that allow separators) include their own, nested lookaheads to ensure the right number of separators are present, before moving on to test the length of the string.

After the positive lookahead validates the length, character set, and number of separators, we can match the individual elements of the ISBN without worrying about their combined length. ‹(?:97[89][-●]?)?› matches the “978” or “979” prefix required by an ISBN-13. The noncapturing group is optional because it will not match within an ISBN-10 subject string. ‹[0-9]{1,5}[-●]?› matches the one to five digit group identifier and an optional, following separator. ‹[0-9]+[-●]?[0-9]+[-●]?› matches the variable-length publisher and title identifiers, along with their optional separators. Finally, ‹[0-9X]$› matches the check digit at the end of the string.

Although a regular expression can check that the final digit uses a valid character (a digit or X), it cannot determine whether it’s correct for the ISBN’s checksum. One of two checksum algorithms (determined by whether you’re working with an ISBN-10 or ISBN-13) are used to provide some level of assurance that the ISBN digits haven’t been accidentally transposed or otherwise entered incorrectly. The JavaScript and Python example code shown earlier implemented both algorithms. The following sections describe the checksum rules in order to help you implement these algorithms with other programming languages.

Step 1:
sum = 10×0 + 9×5 + 8×9 + 7×6 + 6×5 + 5×2 + 4×0 + 3×6 + 2×8
    =    0 +  45 +  72 +  42 +  30 +  10 +   0 +  18 +  16
    = 233
Step 2:
    233 ÷ 11 = 21, remainder 2
Step 3:
    11 − 2 = 9
Step 4:
    9 [no substitution required]

Step 1:
sum = 1×9 + 3×7 + 1×8 + 3×0 + 1×5 + 3×9 + 1×6 + 3×5 + 1×2 + 3×0 + 1×6 + 3×8
    =   9 +  21 +   8 +   0 +   5 +  27 +   6 +  15 +   2 +   0 +   6 +  24
    = 123
Step 2:
    123 ÷ 10 = 12, remainder 3
Step 3:
    10 − 3 = 7
Step 4:
    7 [no substitution required]

The most up-to-date version of the ISBN Users’ Manual, along with tools for validating individual ISBNs and converting between ISBN-10 and ISBN-13, can be found on the International ISBN Agency’s website at http://www.isbn-international.org.

Techniques used in the regular expressions in this recipe are discussed in Chapter 2. Recipe 2.3 explains character classes. Recipe 2.5 explains anchors. Recipe 2.6 explains word boundaries. Recipe 2.8 explains alternation. Recipe 2.9 explains grouping. Recipe 2.12 explains repetition. Recipe 2.16 explains lookaround. Recipe 2.17 explains conditionals.