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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

4.10. Limit the Number of Lines in Text

Problem

You need to check whether a string is composed of five or fewer lines, without regard for how many total characters appear in the string.

Solution

The exact characters or character sequences used as line separators can vary depending on your operating system’s convention, application or user preferences, and so on. Crafting an ideal solution therefore raises questions about what conventions for indicating the start of a new line should be supported. The following solutions support the standard MS-DOS/Windows (\r\n), legacy Mac OS (\r), and Unix/Linux/BSD/OS X (\n) line break conventions.

Regular expression

The following three flavor-specific regexes contain two differences. The first regex uses atomic groups, written as (?>), instead of noncapturing groups, written as (?:), because they have the potential to provide a minor efficiency improvement here for the regex flavors that support them. Python and JavaScript do not support atomic groups, so they are not used with those flavors. The other difference is the tokens used to assert position at the beginning and end of the string (\A or ^ for the beginning of the string, and \z, \Z, or $ for the end). The reasons for this variation are discussed in depth later in this recipe. All three flavor-specific regexes, however, match exactly the same strings:

\A(?>[^\r\n]*(?>\r\n?|\n)){0,4}[^\r\n]*\z
Regex options: None
Regex flavors: .NET, Java, PCRE, Perl, Ruby
\A(?:[^\r\n]*(?:\r\n?|\n)){0,4}[^\r\n]*\Z
Regex options: None
Regex flavor: Python
^(?:[^\r\n]*(?:\r\n?|\n)){0,4}[^\r\n]*$
Regex options: None (“^ and $ match at line breaks” must not be set)
Regex flavor: JavaScript

PHP (PCRE) example

if (preg_match('/\A(?>[^\r\n]*(?>\r\n?|\n)){0,4}[^\r\n]*\z/', 
               $_POST['subject'])) {
    print 'Subject contains five or fewer lines';
} else {
    print 'Subject contains more than five lines';
}

See Recipe 3.6 for help implementing these regular expressions with other programming languages.

Discussion

All of the regular expressions shown so far in this recipe use a grouping that matches any number of non-line-break characters followed by an MS-DOS/Windows, legacy Mac OS, or Unix/Linux/BSD/OS X line break sequence. The grouping is repeated between zero and four times, since four line breaks occur in five lines of text. After the grouping, we allow one last sequence of non-line-break characters to fill out the fifth line, if present.

In the following example, we’ve broken up the first version of the regex into its individual parts. We’ll explain the variations for alternative regex flavors afterward:

\A          # Assert position at the beginning of the string.
(?>         # Group but don't capture or keep backtracking positions:
  [^\r\n]*  #   Match zero or more characters except CR and LF.
  (?>       #   Group but don't capture or keep backtracking positions:
    \r\n?   #     Match a CR, with an optional following LF (CRLF).
   |        #    Or:
    \n      #     Match a standalone LF character.
  )         #   End the noncapturing, atomic group.
){0,4}      # End group; repeat between zero and four times.
[^\r\n]*    # Match zero or more characters except CR and LF.
\z          # Assert position at the end of the string.
Regex options: Free-spacing
Regex flavors: .NET, Java, PCRE, Perl, Ruby

The leading \A matches the position at the beginning of the string, and \z matches at the end. This helps to ensure that the entire string contains no more than five lines, because unless the regex is anchored to the start and end of the text, it can match any five lines within a longer string.

Next, an atomic group (see Recipe 2.14) encloses a character class that matches any number of non-line-break characters and a subgroup that matches one line break sequence. The character class is optional (in that its following quantifier allows it to repeat zero times), but the subgroup is required and must match exactly one line break per repetition of the outer group. The outer group’s immediately following quantifier allows it to repeat between zero and four times. Zero repetitions allows matching a completely empty string, or a string with only one line (no line breaks).

Following the outer group is another character class that matches zero or more non-line-break characters. This lets the regex fill in the match with the fifth line of subject text, if present. We can’t simply omit this class and change the preceding quantifier to {0,5}, because then the text would have to end with a line break to match at all. So long as the last line was empty, it would also allow matching six lines, since six lines are separated by five line breaks. That’s no good.

In all of these regexes, the subgroup matches any of three line break sequences:

  • A carriage return followed by a line feed (\r\n, the conventional MS-DOS/Windows line break sequence)

  • A standalone carriage return (\r, the legacy Mac OS line break character)

  • A standalone line feed (\n, the conventional Unix/Linux/BSD/OS X line break character)

Now let’s move on to the cross-flavor differences.

The first version of the regex (used by all flavors except Python and JavaScript) uses atomic groups rather than simple noncapturing groups. Although in some cases the use of atomic groups can have a much more profound impact, in this case they simply let the regex engine avoid a bit of unnecessary backtracking that can occur if the match attempt fails.

The other cross-flavor differences are the tokens used to assert position at the beginning and end of the string. All of the regex flavors discussed here support ^ and $, so why do some of the regexes use \A, \Z, and \z instead? The short explanation is that the meaning of these metacharacters differs slightly between regular expression flavors. The long explanation leads us to a bit of regex history….

When using Perl to read a line from a file, the resulting string ends with a line break. Hence, Perl introduced an “enhancement” to the traditional meaning of $ that has since been copied by most regex flavors. In addition to matching the absolute end of a string, Perl’s $ matches just before a string-terminating line break. Perl also introduced two more assertions that match the end of a string: \Z and \z. Perl’s \Z anchor has the same quirky meaning as $, except that it doesn’t change when the option to let ^ and $ match at line breaks is set. \z always matches only the absolute end of a string, no exceptions. Since this recipe explicitly deals with line breaks in order to count the lines in a string, it uses the \z assertion for the regex flavors that support it, to ensure that an empty, sixth line is not allowed.

Most of the other regex flavors copied Perl’s end-of-line/string anchors. .NET, Java, PCRE, and Ruby all support both \Z and \z with the same meanings as Perl. Python includes only \Z (uppercase), but confusingly changes its meaning to match only the absolute end of the string, just like Perl’s lowercase \z. JavaScript doesn’t include any “z” anchors, but unlike all of the other flavors discussed here, its $ anchor matches only at the absolute end of the string (when the option to let ^ and $ match at line breaks is not enabled).

As for \A, the situation is somewhat better. It always matches only at the start of a string, and it means exactly the same thing in all flavors discussed here, except JavaScript (which doesn’t support it).

Tip

Although it’s unfortunate that these kinds of confusing cross-flavor inconsistencies exist, one of the benefits of using the regular expressions in this book is that you generally won’t need to worry about them. Gory details like the ones we’ve just described are included in case you care to dig deeper.

Variations

Working with esoteric line separators

The previously shown regexes limit support to the conventional MS-DOS/Windows, Unix/Linux/BSD/OS X, and legacy Mac OS line break character sequences. However, there are several rarer vertical whitespace characters that you might occasionally encounter. The following regexes take these additional characters into account while limiting matches to five lines of text or fewer

\A(?>\V*\R){0,4}\V*\z
Regex options: None
Regex flavors: PCRE 7.2 (with the PCRE_BSR_UNICODE option), Perl 5.10
\A(?>[^\n-\r\x85\x{2028}\x{2029}]*(?>\r\n?|↵
[\n-\f\x85\x{2028}\x{2029}])){0,4}[^\n-\r\x85\x{2028}\x{2029}]*\z
Regex options: None
Regex flavors: Java 7, PCRE, Perl
\A(?>[^\n-\r\u0085\u2028\u2029]*(?>\r\n?|↵
[\n-\f\u0085\u2028\u2029])){0,4}[^\n-\r\u0085\u2028\u2029]*\z
Regex options: None
Regex flavors: .NET, Java, Ruby 1.9
\A(?>[^\n-\r\x85\u2028\u2029]*(?>\r\n?|↵
[\n-\f\x85\u2028\u2029])){0,4}[^\n-\r\x85\u2028\u2029]*\z
Regex options: None
Regex flavors: .NET, Java
\A(?:[^\n-\r\x85\u2028\u2029]*(?:\r\n?|↵
[\n-\f\x85\u2028\u2029])){0,4}[^\n-\r\x85\u2028\u2029]*\Z
Regex options: None
Regex flavor: Python
^(?:[^\n-\r\x85\u2028\u2029]*(?:\r\n?|↵
[\n-\f\x85\u2028\u2029])){0,4}[^\n-\r\x85\u2028\u2029]*$
Regex options: None (“^ and $ match at line breaks” must not be set)
Regex flavor: JavaScript

Ruby 1.8 does not support Unicode regular expressions, and therefore cannot use any of these options. Ruby 1.9 does not support the shorter \xNN syntax for non-ASCII character positions (anything greater than 0x7F), and therefore must use \u0085 instead of \x85.

All of these regexes handle the line separators in Table 4-1, listed with their Unicode positions and names. This list comprises the characters that the Unicode standard recognizes as line terminators.

Table 4-1. Line separators

Unicode sequence

Regex equivalent

Name

Abbr.

Common usage

U+000D U+000A

\r\n

Carriage return and line feed

CRLF

Windows and MS-DOS text files

U+000A

\n

Line feed

LF

Unix, Linux, BSD, and OS X text files

U+000B

\v or \x0B

Line tabulation (aka vertical tab)

VT

(Rare)

U+000C

\f

Form feed

FF

(Rare)

U+000D

\r

Carriage return

CR

Legacy Mac OS text files

U+0085

\x85 or \u0085

Next line

NEL

IBM mainframe text files

U+2028

\u2028 or \x{2028}

Line separator

LS

(Rare)

U+2029

\u2029 or \x{2029}

Paragraph separator

PS

(Rare)

See Also

Recipe 4.9 shows how to limit the length of text based on characters and words, rather than lines.

Techniques used in the regular expressions in this recipe are discussed in Chapter 2. Recipe 2.2 explains how to match nonprinting characters. Recipe 2.3 explains character classes. Recipe 2.5 explains anchors. Recipe 2.7 explains how to match Unicode characters. Recipe 2.9 explains grouping. Recipe 2.12 explains repetition. Recipe 2.14 explains atomic groups.

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