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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

4.4. Validate Traditional Date Formats

Problem

You want to validate dates in the traditional formats mm/dd/yy, mm/dd/yyyy, dd/mm/yy, and dd/mm/yyyy. You want to use a simple regex that simply checks whether the input looks like a date, without trying to weed out things such as February 31st.

Solution

Solution 1: Match any of these date formats, allowing leading zeros to be omitted:

^[0-3]?[0-9]/[0-3]?[0-9]/(?:[0-9]{2})?[0-9]{2}$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Solution 2: Match any of these date formats, requiring leading zeros:

^[0-3][0-9]/[0-3][0-9]/(?:[0-9][0-9])?[0-9][0-9]$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Solution 3: Match m/d/yy and mm/dd/yyyy, allowing any combination of one or two digits for the day and month, and two or four digits for the year:

^(1[0-2]|0?[1-9])/(3[01]|[12][0-9]|0?[1-9])/(?:[0-9]{2})?[0-9]{2}$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Solution 4: Match mm/dd/yyyy, requiring leading zeros:

^(1[0-2]|0[1-9])/(3[01]|[12][0-9]|0[1-9])/[0-9]{4}$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Solution 5: Match d/m/yy and dd/mm/yyyy, allowing any combination of one or two digits for the day and month, and two or four digits for the year:

^(3[01]|[12][0-9]|0?[1-9])/(1[0-2]|0?[1-9])/(?:[0-9]{2})?[0-9]{2}$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Solution 6: Match dd/mm/yyyy, requiring leading zeros:

^(3[01]|[12][0-9]|0[1-9])/(1[0-2]|0[1-9])/[0-9]{4}$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Solution 7: Match any of these date formats with greater accuracy, allowing leading zeros to be omitted:

^(?:(1[0-2]|0?[1-9])/(3[01]|[12][0-9]|0?[1-9])|↵
(3[01]|[12][0-9]|0?[1-9])/(1[0-2]|0?[1-9]))/(?:[0-9]{2})?[0-9]{2}$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

We can use the free-spacing option to make this regular expression easier to read:

^(?:
  # m/d or mm/dd
  (1[0-2]|0?[1-9])/(3[01]|[12][0-9]|0?[1-9])
|
  # d/m or dd/mm
  (3[01]|[12][0-9]|0?[1-9])/(1[0-2]|0?[1-9])
)
# /yy or /yyyy
/(?:[0-9]{2})?[0-9]{2}$
Regex options: Free-spacing
Regex flavors: .NET, Java, XRegExp, PCRE, Perl, Python, Ruby

Solution 8: Match any of these date formats with greater accuracy, requiring leading zeros:

^(?:(1[0-2]|0[1-9])/(3[01]|[12][0-9]|0[1-9])|↵
(3[01]|[12][0-9]|0[1-9])/(1[0-2]|0[1-9]))/[0-9]{4}$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

The same solution using the free-spacing option to make it easier to read:

^(?:
  # mm/dd
  (1[0-2]|0[1-9])/(3[01]|[12][0-9]|0[1-9])
|
  # dd/mm
  (3[01]|[12][0-9]|0[1-9])/(1[0-2]|0[1-9])
)
# /yyyy
/[0-9]{4}$
Regex options: Free-spacing
Regex flavors: .NET, Java, XRegExp, PCRE, Perl, Python, Ruby

Discussion

You might think that something as conceptually trivial as a date should be an easy job for a regular expression. But it isn’t, for two reasons. Because dates are such an everyday thing, humans are very sloppy with them. 4/1 may be April Fools’ Day to you. To somebody else, it may be the first working day of the year, if New Year’s Day is on a Friday.

The other issue is that regular expressions don’t deal directly with numbers. You can’t tell a regular expression to “match a number between 1 and 31”, for instance. Regular expressions work character by character. We use 3[01]|[12][0-9]|0?[1-9] to match 3 followed by 0 or 1, or to match 1 or 2 followed by any digit, or to match an optional 0 followed by 1 to 9. In character classes, we can use ranges for single digits, such as [1-9]. That’s because the characters for the digits 0 through 9 occupy consecutive positions in the ASCII and Unicode character tables. See Chapter 6 for more details on matching all kinds of numbers with regular expressions.

Because of this, you have to choose how simple or how accurate you want your regular expression to be. If you already know your subject text doesn’t contain any invalid dates, you could use a trivial regex such as \d{2}/\d{2}/\d{4}. The fact that this matches things like 99/99/9999 is irrelevant if those don’t occur in the subject text.

The first two solutions for this recipe are quick and simple, too, and they also match invalid dates, such as 0/0/00 and 31/31/2008. They only use literal characters for the date delimiters, character classes (see Recipe 2.3) for the digits, and the question mark (see Recipe 2.12) to make certain digits optional. (?:[0-9]{2})?[0-9]{2} allows the year to consist of two or four digits. [0-9]{2} matches exactly two digits. (?:[0-9]{2})? matches zero or two digits. The noncapturing group (see Recipe 2.9) is required, because the question mark needs to apply to the character class and the quantifier {2} combined. [0-9]{2}? matches exactly two digits, just like [0-9]{2}. Without the group, the question mark makes the quantifier lazy, which has no effect because {2} cannot repeat more than two times or fewer than two times.

Solutions 3 through 6 restrict the month to numbers between 1 and 12, and the day to numbers between 1 and 31. We use alternation (see Recipe 2.8) inside a group to match various pairs of digits to form a range of two-digit numbers. We use capturing groups here because you’ll probably want to capture the day and month numbers anyway.

The final two solutions are a little more complex, so we’re presenting these in both condensed and free-spacing form. The only difference between the two forms is readability. JavaScript does not support free-spacing. The final two solutions allow all of the date formats, just like the first two examples. The difference is that the last two use an extra level of alternation to restrict the dates to 12/31 and 31/12, disallowing invalid months, such as 31/31.

Variations

If you want to search for dates in larger bodies of text instead of checking whether the input as a whole is a date, you cannot use the anchors ^ and $. Merely removing the anchors from the regular expression is not the right solution. That would allow any of these regexes to match 12/12/2001 within 9912/12/200199, for example. Instead of anchoring the regex match to the start and end of the subject, you have to specify that the date cannot be part of longer sequences of digits.

This is easily done with a pair of word boundaries. In regular expressions, digits are treated as characters that can be part of words. Replace both ^ and $ with \b. As an example:

\b(1[0-2]|0[1-9])/(3[01]|[12][0-9]|0[1-9])/[0-9]{4}\b
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

See Also

This chapter has several other recipes for matching dates and times. Recipe 4.5 shows how to validate traditional date formats more accurately. Recipe 4.6 shows how to validate traditional time formats. Recipe 4.7 shows how to validate date and time formats according to the ISO 8601 standard.

Recipe 6.7 explains how you can create a regular expression to match a number in a given range of numbers.

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 2.8 explains alternation. Recipe 2.9 explains grouping. Recipe 2.12 explains repetition.

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