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Regular Expressions Cookbook, 2nd Edition

Cover of Regular Expressions Cookbook, 2nd Edition by Jan Goyvaerts... Published by O'Reilly Media, Inc.
  1. Regular Expressions Cookbook
  2. Preface
    1. Caught in the Snarls of Different Versions
    2. Intended Audience
    3. Technology Covered
    4. Organization of This Book
    5. Conventions Used in This Book
    6. Using Code Examples
    7. Safari® Books Online
    8. How to Contact Us
    9. Acknowledgments
  3. 1. Introduction to Regular Expressions
    1. Regular Expressions Defined
      1. Many Flavors of Regular Expressions
      2. Regex Flavors Covered by This Book
    2. Search and Replace with Regular Expressions
      1. Many Flavors of Replacement Text
    3. Tools for Working with Regular Expressions
      1. RegexBuddy
      2. RegexPal
      3. RegexMagic
      4. More Online Regex Testers
      5. More Desktop Regular Expression Testers
      6. grep
      7. Popular Text Editors
  4. 2. Basic Regular Expression Skills
    1. 2.1. Match Literal Text
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    2. 2.2. Match Nonprintable Characters
      1. Problem
      2. Solution
      3. Discussion
      4. Variations on Representations of Nonprinting Characters
      5. See Also
    3. 2.3. Match One of Many Characters
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. Flavor-Specific Features
      6. See Also
    4. 2.4. Match Any Character
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    5. 2.5. Match Something at the Start and/or the End of a Line
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    6. 2.6. Match Whole Words
      1. Problem
      2. Solution
      3. Discussion
      4. Word Characters
      5. See Also
    7. 2.7. Unicode Code Points, Categories, Blocks, and Scripts
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    8. 2.8. Match One of Several Alternatives
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    9. 2.9. Group and Capture Parts of the Match
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    10. 2.10. Match Previously Matched Text Again
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    11. 2.11. Capture and Name Parts of the Match
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    12. 2.12. Repeat Part of the Regex a Certain Number of Times
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    13. 2.13. Choose Minimal or Maximal Repetition
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    14. 2.14. Eliminate Needless Backtracking
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    15. 2.15. Prevent Runaway Repetition
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    16. 2.16. Test for a Match Without Adding It to the Overall Match
      1. Problem
      2. Solution
      3. Discussion
      4. Alternative to Lookbehind
      5. Solution Without Lookbehind
      6. See Also
    17. 2.17. Match One of Two Alternatives Based on a Condition
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    18. 2.18. Add Comments to a Regular Expression
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
    19. 2.19. Insert Literal Text into the Replacement Text
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    20. 2.20. Insert the Regex Match into the Replacement Text
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    21. 2.21. Insert Part of the Regex Match into the Replacement Text
      1. Problem
      2. Solution
      3. Discussion
      4. Solution Using Named Capture
      5. See Also
    22. 2.22. Insert Match Context into the Replacement Text
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  5. 3. Programming with Regular Expressions
    1. Programming Languages and Regex Flavors
      1. Languages Covered in This Chapter
      2. More Programming Languages
    2. 3.1. Literal Regular Expressions in Source Code
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    3. 3.2. Import the Regular Expression Library
      1. Problem
      2. Solution
      3. Discussion
    4. 3.3. Create Regular Expression Objects
      1. Problem
      2. Solution
      3. Discussion
      4. Compiling a Regular Expression Down to CIL
      5. Discussion
      6. See Also
    5. 3.4. Set Regular Expression Options
      1. Problem
      2. Solution
      3. Discussion
      4. Additional Language-Specific Options
      5. See Also
    6. 3.5. Test If a Match Can Be Found Within a Subject String
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    7. 3.6. Test Whether a Regex Matches the Subject String Entirely
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 3.7. Retrieve the Matched Text
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    9. 3.8. Determine the Position and Length of the Match
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    10. 3.9. Retrieve Part of the Matched Text
      1. Problem
      2. Solution
      3. Discussion
      4. Named Capture
      5. See Also
    11. 3.10. Retrieve a List of All Matches
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    12. 3.11. Iterate over All Matches
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    13. 3.12. Validate Matches in Procedural Code
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    14. 3.13. Find a Match Within Another Match
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    15. 3.14. Replace All Matches
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    16. 3.15. Replace Matches Reusing Parts of the Match
      1. Problem
      2. Solution
      3. Discussion
      4. Named Capture
      5. See Also
    17. 3.16. Replace Matches with Replacements Generated in Code
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    18. 3.17. Replace All Matches Within the Matches of Another Regex
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    19. 3.18. Replace All Matches Between the Matches of Another Regex
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    20. 3.19. Split a String
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    21. 3.20. Split a String, Keeping the Regex Matches
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    22. 3.21. Search Line by Line
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    23. Construct a Parser
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  6. 4. Validation and Formatting
    1. 4.1. Validate Email Addresses
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    2. 4.2. Validate and Format North American Phone Numbers
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    3. 4.3. Validate International Phone Numbers
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    4. 4.4. Validate Traditional Date Formats
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    5. 4.5. Validate Traditional Date Formats, Excluding Invalid Dates
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    6. 4.6. Validate Traditional Time Formats
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    7. 4.7. Validate ISO 8601 Dates and Times
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 4.8. Limit Input to Alphanumeric Characters
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    9. 4.9. Limit the Length of Text
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    10. 4.10. Limit the Number of Lines in Text
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    11. 4.11. Validate Affirmative Responses
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    12. 4.12. Validate Social Security Numbers
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    13. 4.13. Validate ISBNs
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    14. 4.14. Validate ZIP Codes
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    15. 4.15. Validate Canadian Postal Codes
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    16. 4.16. Validate U.K. Postcodes
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    17. 4.17. Find Addresses with Post Office Boxes
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    18. 4.18. Reformat Names From “FirstName LastName” to “LastName, FirstName”
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    19. 4.19. Validate Password Complexity
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    20. 4.20. Validate Credit Card Numbers
      1. Problem
      2. Solution
      3. Discussion
      4. Extra Validation with the Luhn Algorithm
      5. See Also
    21. 4.21. European VAT Numbers
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
  7. 5. Words, Lines, and Special Characters
    1. 5.1. Find a Specific Word
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    2. 5.2. Find Any of Multiple Words
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    3. 5.3. Find Similar Words
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 5.4. Find All Except a Specific Word
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    5. 5.5. Find Any Word Not Followed by a Specific Word
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    6. 5.6. Find Any Word Not Preceded by a Specific Word
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    7. 5.7. Find Words Near Each Other
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    8. 5.8. Find Repeated Words
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    9. 5.9. Remove Duplicate Lines
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    10. 5.10. Match Complete Lines That Contain a Word
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    11. 5.11. Match Complete Lines That Do Not Contain a Word
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    12. 5.12. Trim Leading and Trailing Whitespace
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    13. 5.13. Replace Repeated Whitespace with a Single Space
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    14. 5.14. Escape Regular Expression Metacharacters
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
  8. 6. Numbers
    1. 6.1. Integer Numbers
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    2. 6.2. Hexadecimal Numbers
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    3. 6.3. Binary Numbers
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 6.4. Octal Numbers
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 6.5. Decimal Numbers
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. 6.6. Strip Leading Zeros
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    7. 6.7. Numbers Within a Certain Range
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 6.8. Hexadecimal Numbers Within a Certain Range
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    9. 6.9. Integer Numbers with Separators
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    10. 6.10. Floating-Point Numbers
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    11. 6.11. Numbers with Thousand Separators
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    12. 6.12. Add Thousand Separators to Numbers
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    13. 6.13. Roman Numerals
      1. Problem
      2. Solution
      3. Discussion
      4. Convert Roman Numerals to Decimal
      5. See Also
  9. 7. Source Code and Log Files
    1. Keywords
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    2. Identifiers
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    3. Numeric Constants
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. Operators
      1. Problem
      2. Solution
      3. Discussion
    5. Single-Line Comments
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. Multiline Comments
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    7. All Comments
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. Strings
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    9. Strings with Escapes
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    10. Regex Literals
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    11. Here Documents
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    12. Common Log Format
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    13. Combined Log Format
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    14. Broken Links Reported in Web Logs
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  10. 8. URLs, Paths, and Internet Addresses
    1. 8.1. Validating URLs
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    2. 8.2. Finding URLs Within Full Text
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    3. 8.3. Finding Quoted URLs in Full Text
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 8.4. Finding URLs with Parentheses in Full Text
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 8.5. Turn URLs into Links
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. 8.6. Validating URNs
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    7. 8.7. Validating Generic URLs
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 8.8. Extracting the Scheme from a URL
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    9. 8.9. Extracting the User from a URL
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    10. 8.10. Extracting the Host from a URL
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    11. 8.11. Extracting the Port from a URL
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    12. 8.12. Extracting the Path from a URL
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    13. 8.13. Extracting the Query from a URL
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    14. 8.14. Extracting the Fragment from a URL
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    15. 8.15. Validating Domain Names
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    16. 8.16. Matching IPv4 Addresses
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    17. 8.17. Matching IPv6 Addresses
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    18. 8.18. Validate Windows Paths
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    19. 8.19. Split Windows Paths into Their Parts
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    20. 8.20. Extract the Drive Letter from a Windows Path
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    21. 8.21. Extract the Server and Share from a UNC Path
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    22. 8.22. Extract the Folder from a Windows Path
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    23. 8.23. Extract the Filename from a Windows Path
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    24. 8.24. Extract the File Extension from a Windows Path
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    25. 8.25. Strip Invalid Characters from Filenames
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  11. 9. Markup and Data Formats
    1. Processing Markup and Data Formats with Regular Expressions
      1. Basic Rules for Formats Covered in This Chapter
    2. 9.1. Find XML-Style Tags
      1. Problem
      2. Solution
      3. Discussion
      4. Skip Tricky (X)HTML and XML Sections
      5. See Also
    3. 9.2. Replace <b> Tags with <strong>
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    4. 9.3. Remove All XML-Style Tags Except <em> and <strong>
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    5. 9.4. Match XML Names
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    6. 9.5. Convert Plain Text to HTML by Adding <p> and <br> Tags
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    7. 9.6. Decode XML Entities
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 9.7. Find a Specific Attribute in XML-Style Tags
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    9. 9.8. Add a cellspacing Attribute to <table> Tags That Do Not Already Include It
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    10. 9.9. Remove XML-Style Comments
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    11. 9.10. Find Words Within XML-Style Comments
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    12. 9.11. Change the Delimiter Used in CSV Files
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    13. 9.12. Extract CSV Fields from a Specific Column
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    14. 9.13. Match INI Section Headers
      1. Problem
      2. Solution
      3. Discussion
      4. Variations
      5. See Also
    15. 9.14. Match INI Section Blocks
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    16. 9.15. Match INI Name-Value Pairs
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  12. Index
  13. About the Authors
  14. Colophon
  15. Copyright
O'Reilly logo

4.13. Validate ISBNs

Problem

You need to check the validity of an International Standard Book Number (ISBN), which can be in either the older ISBN-10 or the current ISBN-13 format. You want to allow a leading “ISBN” identifier, and ISBN parts can optionally be separated by hyphens or spaces. All of the following are examples of valid input:

  • ISBN 978-0-596-52068-7

  • ISBN-13: 978-0-596-52068-7

  • 978 0 596 52068 7

  • 9780596520687

  • ISBN-10 0-596-52068-9

  • 0-596-52068-9

Solution

You cannot validate an ISBN using a regex alone, because the last digit is computed using a checksum algorithm. The regular expressions in this section validate the format of an ISBN, whereas the subsequent code examples include a validity check for the final digit.

Regular expressions

Three regex solutions follow that allow you to match ISBN-10s and ISBN-13s, either exclusively or together. Each of the solutions is shown with and without free-spacing and comments. JavaScript doesn’t support free-spacing, but with other programming languages you can choose whichever suits you best.

In the free-spaced regexes, literal space characters have been escaped with backslashes. Java’s free-spacing mode requires that even spaces within character classes be escaped.

ISBN-10:

^(?:ISBN(?:-10)?:?)?(?=[0-9X]{10}$|(?=(?:[0-9]+[-]){3})[-0-9X]{13}$)↵
[0-9]{1,5}[-]?[0-9]+[-]?[0-9]+[-]?[0-9X]$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
^
(?:ISBN(?:-10)?:?\ )?     # Optional ISBN/ISBN-10 identifier.
(?=                       # Basic format pre-checks (lookahead):
  [0-9X]{10}$             #   Require 10 digits/Xs (no separators).
 |                        #  Or:
  (?=(?:[0-9]+[-\ ]){3})  #   Require 3 separators
  [-\ 0-9X]{13}$          #     out of 13 characters total.
)                         # End format pre-checks.
[0-9]{1,5}[-\ ]?          # 1-5 digit group identifier.
[0-9]+[-\ ]?[0-9]+[-\ ]?  # Publisher and title identifiers.
[0-9X]                    # Check digit.
$
Regex options: Free-spacing
Regex flavors: .NET, Java, XRegExp, PCRE, Perl, Python, Ruby

ISBN-13:

^(?:ISBN(?:-13)?:?)?(?=[0-9]{13}$|(?=(?:[0-9]+[-]){4})[-0-9]{17}$)↵
97[89][-]?[0-9]{1,5}[-]?[0-9]+[-]?[0-9]+[-]?[0-9]$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
^
(?:ISBN(?:-13)?:?\ )?     # Optional ISBN/ISBN-13 identifier.
(?=                       # Basic format pre-checks (lookahead):
  [0-9]{13}$              #   Require 13 digits (no separators).
 |                        #  Or:
  (?=(?:[0-9]+[-\ ]){4})  #   Require 4 separators
  [-\ 0-9]{17}$           #     out of 17 characters total.
)                         # End format pre-checks.
97[89][-\ ]?              # ISBN-13 prefix.
[0-9]{1,5}[-\ ]?          # 1-5 digit group identifier.
[0-9]+[-\ ]?[0-9]+[-\ ]?  # Publisher and title identifiers.
[0-9]                     # Check digit.
$
Regex options: Free-spacing
Regex flavors: .NET, Java, XRegExp, PCRE, Perl, Python, Ruby

ISBN-10 or ISBN-13:

^(?:ISBN(?:-1[03])?:?)?(?=[0-9X]{10}$|(?=(?:[0-9]+[-]){3})↵
[-0-9X]{13}$|97[89][0-9]{10}$|(?=(?:[0-9]+[-]){4})[-0-9]{17}$)↵
(?:97[89][-]?)?[0-9]{1,5}[-]?[0-9]+[-]?[0-9]+[-]?[0-9X]$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
^
(?:ISBN(?:-1[03])?:?\ )?  # Optional ISBN/ISBN-10/ISBN-13 identifier.
(?=                       # Basic format pre-checks (lookahead):
  [0-9X]{10}$             #   Require 10 digits/Xs (no separators).
 |                        #  Or:
  (?=(?:[0-9]+[-\ ]){3})  #   Require 3 separators
  [-\ 0-9X]{13}$          #     out of 13 characters total.
 |                        #  Or:
  97[89][0-9]{10}$        #   978/979 plus 10 digits (13 total).
 |                        #  Or:
  (?=(?:[0-9]+[-\ ]){4})  #   Require 4 separators
  [-\ 0-9]{17}$           #     out of 17 characters total.
)                         # End format pre-checks.
(?:97[89][-\ ]?)?         # Optional ISBN-13 prefix.
[0-9]{1,5}[-\ ]?          # 1-5 digit group identifier.
[0-9]+[-\ ]?[0-9]+[-\ ]?  # Publisher and title identifiers.
[0-9X]                    # Check digit.
$
Regex options: Free-spacing
Regex flavors: .NET, Java, XRegExp, PCRE, Perl, Python, Ruby

JavaScript example, with checksum validation

var subject = document.getElementById("isbn").value;

// Checks for ISBN-10 or ISBN-13 format
var regex = /^(?:ISBN(?:-1[03])?:? )?(?=[0-9X]{10}$|↵
(?=(?:[0-9]+[- ]){3})[- 0-9X]{13}$|97[89][0-9]{10}$|↵
(?=(?:[0-9]+[- ]){4})[- 0-9]{17}$)(?:97[89][- ]?)?[0-9]{1,5}[- ]?↵
[0-9]+[- ]?[0-9]+[- ]?[0-9X]$/;

if (regex.test(subject)) {
    // Remove non ISBN digits, then split into an array
    var chars = subject.replace(/[- ]|^ISBN(?:-1[03])?:?/g, "").split("");
    // Remove the final ISBN digit from `chars`, and assign it to `last`
    var last = chars.pop();
    var sum = 0;
    var check, i;

    if (chars.length == 9) {
        // Compute the ISBN-10 check digit
        chars.reverse();
        for (i = 0; i < chars.length; i++) {
            sum += (i + 2) * parseInt(chars[i], 10);
        }
        check = 11 - (sum % 11);
        if (check == 10) {
            check = "X";
        } else if (check == 11) {
            check = "0";
        }
    } else {
        // Compute the ISBN-13 check digit
        for (i = 0; i < chars.length; i++) {
            sum += (i % 2 * 2 + 1) * parseInt(chars[i], 10);
        }
        check = 10 - (sum % 10);
        if (check == 10) {
            check = "0";
        }
    }

    if (check == last) {
        alert("Valid ISBN");
    } else {
        alert("Invalid ISBN check digit");
    }
} else {
    alert("Invalid ISBN");
}

Python example, with checksum validation

import re
import sys

subject = sys.argv[1]

# Checks for ISBN-10 or ISBN-13 format
regex = re.compile("^(?:ISBN(?:-1[03])?:? )?(?=[0-9X]{10}$|↵
(?=(?:[0-9]+[- ]){3})[- 0-9X]{13}$|97[89][0-9]{10}$|↵
(?=(?:[0-9]+[- ]){4})[- 0-9]{17}$)(?:97[89][- ]?)?[0-9]{1,5}[- ]?↵
[0-9]+[- ]?[0-9]+[- ]?[0-9X]$")

if regex.search(subject):
    # Remove non ISBN digits, then split into a list
    chars = list(re.sub("[- ]|^ISBN(?:-1[03])?:?", "", subject))
    # Remove the final ISBN digit from `chars`, and assign it to `last`
    last = chars.pop()

    if len(chars) == 9:
        # Compute the ISBN-10 check digit
        val = sum((x + 2) * int(y) for x,y in enumerate(reversed(chars)))
        check = 11 - (val % 11)
        if check == 10:
            check = "X"
        elif check == 11:
            check = "0"
    else:
        # Compute the ISBN-13 check digit
        val = sum((x % 2 * 2 + 1) * int(y) for x,y in enumerate(chars))
        check = 10 - (val % 10)
        if check == 10:
            check = "0"

    if (str(check) == last):
        print("Valid ISBN")
    else:
        print("Invalid ISBN check digit")
else:
    print("Invalid ISBN")

Discussion

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:

  • 13-digit ISBNs start with the prefix 978 or 979.

  • The group identifier identifies the language-sharing country group. It ranges from one to five digits long.

  • The publisher identifier varies in length and is assigned by the national ISBN agency.

  • The title identifier also varies in length and is selected by the publisher.

  • The final character is called the check digit, and is computed using a checksum algorithm. An ISBN-10 check digit can be either a number from 0 to 9 or the letter X (Roman numeral for 10), whereas an ISBN-13 check digit ranges from 0 to 9. The allowed characters are different because the two ISBN types use different checksum algorithms.

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):

  • ISBN

  • ISBN-10

  • ISBN-13

  • ISBN:

  • ISBN-10:

  • ISBN-13:

  • The empty string (no prefix)

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:

[0-9X]{10}$

Allows an ISBN-10 with no separators (10 total characters)

(?=(?:[0-9]+[-]){3})[-0-9X]{13}$

Allows an ISBN-10 with three separators (13 total characters)

97[89][0-9]{10}$

Allows an ISBN-13 with no separators (13 total characters)

(?=(?:[0-9]+[-]){4})[-0-9]{17}$

Allows an ISBN-13 with four separators (17 total characters)

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.

ISBN-10 checksum

The check digit for an ISBN-10 number ranges from 0 to 10 (with the Roman numeral X used instead of 10). It is computed as follows:

  1. Multiply each of the first 9 digits by a number in the descending sequence from 10 to 2, and sum the results.

  2. Divide the sum by 11.

  3. Subtract the remainder (not the quotient) from 11.

  4. If the result is 11, use the number 0; if 10, use the letter X.

Here’s an example of how to derive the ISBN-10 check digit for 0-596-52068-?:

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]

The check digit is 9, so the complete sequence is ISBN 0-596-52068-9.

ISBN-13 checksum

An ISBN-13 check digit ranges from 0 to 9, and is computed using similar steps:

  1. Multiply each of the first 12 digits by 1 or 3, alternating as you move from left to right, and sum the results.

  2. Divide the sum by 10.

  3. Subtract the remainder (not the quotient) from 10.

  4. If the result is 10, use the number 0.

For example, the ISBN-13 check digit for 978-0-596-52068-? is calculated as follows:

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 check digit is 7, and the complete sequence is ISBN 978-0-596-52068-7.

Variations

Find ISBNs in documents

This adaptation of the “ISBN-10 or ISBN-13” regex uses word boundaries instead of anchors to help you find ISBNs within longer text while ensuring that they stand on their own. The “ISBN” identifier has also been made a required string in this version, for two reasons. First, requiring it helps eliminate false positives (without it, the regex could potentially match any 10- or 13-digit number), and second, ISBNs are officially required to use this identifier when printed:

\bISBN(?:-1[03])?:?(?=[0-9X]{10}$|(?=(?:[0-9]+[-]){3})↵
[-0-9X]{13}$|97[89][0-9]{10}$|(?=(?:[0-9]+[-]){4})[-0-9]{17}$)↵
(?:97[89][-]?)?[0-9]{1,5}[-]?[0-9]+[-]?[0-9]+[-]?[0-9X]\b
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Eliminate incorrect ISBN identifiers

A limitation of the previous regexes is that they allow matching an ISBN-10 number preceded by the “ISBN-13” identifier, and vice versa. The following regex uses conditionals (see Recipe 2.17) to ensure that an “ISBN-10” or “ISBN-13” identifier is followed by the appropriate ISBN type. It allows both ISBN-10 and ISBN-13 numbers when the type is not explicitly specified. This regex is overkill in most circumstances because the same result could be achieved more manageably using the ISBN-10 and ISBN-13 specific regexes that were shown earlier, one at a time. It’s included here merely to demonstrate an interesting use of regular expressions:

^
(?:ISBN(-1(?:(0)|3))?:?\ )?
(?(1)
  (?(2)
    # ISBN-10
    (?=[0-9X]{10}$|(?=(?:[0-9]+[- ]){3})[- 0-9X]{13}$)
    [0-9]{1,5}[- ]?[0-9]+[- ]?[0-9]+[- ]?[0-9X]
   |
    # ISBN-13
    (?=[0-9]{13}$|(?=(?:[0-9]+[- ]){4})[- 0-9]{17}$)
    97[89][- ]?[0-9]{1,5}[- ]?[0-9]+[- ]?[0-9]+[- ]?[0-9]
  )
 |
  # No explicit identifier; allow ISBN-10 or ISBN-13
  (?=[0-9X]{10}$|(?=(?:[0-9]+[- ]){3})[- 0-9X]{13}$|97[89][0-9]{10}$|
    (?=(?:[0-9]+[- ]){4})[- 0-9]{17}$)
  (?:97[89][- ]?)?[0-9]{1,5}[- ]?[0-9]+[- ]?[0-9]+[- ]?[0-9X]
)
$
Regex options: Free-spacing
Regex flavors: .NET, PCRE, Perl, Python

See Also

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.

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