Table of Contents for
Regular Expressions Cookbook, 2nd Edition

Version ebook / Retour

Cover image for bash Cookbook, 2nd Edition Regular Expressions Cookbook, 2nd Edition by Steven Levithan Published by O'Reilly Media, Inc., 2012
  1. Cover
  2. Regular Expressions Cookbook
  3. Preface
  4. Caught in the Snarls of Different Versions
  5. Intended Audience
  6. Technology Covered
  7. Organization of This Book
  8. Conventions Used in This Book
  9. Using Code Examples
  10. Safari® Books Online
  11. How to Contact Us
  12. Acknowledgments
  13. 1. Introduction to Regular Expressions
  14. Regular Expressions Defined
  15. Search and Replace with Regular Expressions
  16. Tools for Working with Regular Expressions
  17. 2. Basic Regular Expression Skills
  18. 2.1. Match Literal Text
  19. 2.2. Match Nonprintable Characters
  20. 2.3. Match One of Many Characters
  21. 2.4. Match Any Character
  22. 2.5. Match Something at the Start and/or the End of a Line
  23. 2.6. Match Whole Words
  24. 2.7. Unicode Code Points, Categories, Blocks, and Scripts
  25. 2.8. Match One of Several Alternatives
  26. 2.9. Group and Capture Parts of the Match
  27. 2.10. Match Previously Matched Text Again
  28. 2.11. Capture and Name Parts of the Match
  29. 2.12. Repeat Part of the Regex a Certain Number of Times
  30. 2.13. Choose Minimal or Maximal Repetition
  31. 2.14. Eliminate Needless Backtracking
  32. 2.15. Prevent Runaway Repetition
  33. 2.16. Test for a Match Without Adding It to the Overall Match
  34. 2.17. Match One of Two Alternatives Based on a Condition
  35. 2.18. Add Comments to a Regular Expression
  36. 2.19. Insert Literal Text into the Replacement Text
  37. 2.20. Insert the Regex Match into the Replacement Text
  38. 2.21. Insert Part of the Regex Match into the Replacement Text
  39. 2.22. Insert Match Context into the Replacement Text
  40. 3. Programming with Regular Expressions
  41. Programming Languages and Regex Flavors
  42. 3.1. Literal Regular Expressions in Source Code
  43. 3.2. Import the Regular Expression Library
  44. 3.3. Create Regular Expression Objects
  45. 3.4. Set Regular Expression Options
  46. 3.5. Test If a Match Can Be Found Within a Subject String
  47. 3.6. Test Whether a Regex Matches the Subject String Entirely
  48. 3.7. Retrieve the Matched Text
  49. 3.8. Determine the Position and Length of the Match
  50. 3.9. Retrieve Part of the Matched Text
  51. 3.10. Retrieve a List of All Matches
  52. 3.11. Iterate over All Matches
  53. 3.12. Validate Matches in Procedural Code
  54. 3.13. Find a Match Within Another Match
  55. 3.14. Replace All Matches
  56. 3.15. Replace Matches Reusing Parts of the Match
  57. 3.16. Replace Matches with Replacements Generated in Code
  58. 3.17. Replace All Matches Within the Matches of Another Regex
  59. 3.18. Replace All Matches Between the Matches of Another Regex
  60. 3.19. Split a String
  61. 3.20. Split a String, Keeping the Regex Matches
  62. 3.21. Search Line by Line
  63. Construct a Parser
  64. 4. Validation and Formatting
  65. 4.1. Validate Email Addresses
  66. 4.2. Validate and Format North American Phone Numbers
  67. 4.3. Validate International Phone Numbers
  68. 4.4. Validate Traditional Date Formats
  69. 4.5. Validate Traditional Date Formats, Excluding Invalid Dates
  70. 4.6. Validate Traditional Time Formats
  71. 4.7. Validate ISO 8601 Dates and Times
  72. 4.8. Limit Input to Alphanumeric Characters
  73. 4.9. Limit the Length of Text
  74. 4.10. Limit the Number of Lines in Text
  75. 4.11. Validate Affirmative Responses
  76. 4.12. Validate Social Security Numbers
  77. 4.13. Validate ISBNs
  78. 4.14. Validate ZIP Codes
  79. 4.15. Validate Canadian Postal Codes
  80. 4.16. Validate U.K. Postcodes
  81. 4.17. Find Addresses with Post Office Boxes
  82. 4.18. Reformat Names From “FirstName LastName” to “LastName, FirstName”
  83. 4.19. Validate Password Complexity
  84. 4.20. Validate Credit Card Numbers
  85. 4.21. European VAT Numbers
  86. 5. Words, Lines, and Special Characters
  87. 5.1. Find a Specific Word
  88. 5.2. Find Any of Multiple Words
  89. 5.3. Find Similar Words
  90. 5.4. Find All Except a Specific Word
  91. 5.5. Find Any Word Not Followed by a Specific Word
  92. 5.6. Find Any Word Not Preceded by a Specific Word
  93. 5.7. Find Words Near Each Other
  94. 5.8. Find Repeated Words
  95. 5.9. Remove Duplicate Lines
  96. 5.10. Match Complete Lines That Contain a Word
  97. 5.11. Match Complete Lines That Do Not Contain a Word
  98. 5.12. Trim Leading and Trailing Whitespace
  99. 5.13. Replace Repeated Whitespace with a Single Space
  100. 5.14. Escape Regular Expression Metacharacters
  101. 6. Numbers
  102. 6.1. Integer Numbers
  103. 6.2. Hexadecimal Numbers
  104. 6.3. Binary Numbers
  105. 6.4. Octal Numbers
  106. 6.5. Decimal Numbers
  107. 6.6. Strip Leading Zeros
  108. 6.7. Numbers Within a Certain Range
  109. 6.8. Hexadecimal Numbers Within a Certain Range
  110. 6.9. Integer Numbers with Separators
  111. 6.10. Floating-Point Numbers
  112. 6.11. Numbers with Thousand Separators
  113. 6.12. Add Thousand Separators to Numbers
  114. 6.13. Roman Numerals
  115. 7. Source Code and Log Files
  116. Keywords
  117. Identifiers
  118. Numeric Constants
  119. Operators
  120. Single-Line Comments
  121. Multiline Comments
  122. All Comments
  123. Strings
  124. Strings with Escapes
  125. Regex Literals
  126. Here Documents
  127. Common Log Format
  128. Combined Log Format
  129. Broken Links Reported in Web Logs
  130. 8. URLs, Paths, and Internet Addresses
  131. 8.1. Validating URLs
  132. 8.2. Finding URLs Within Full Text
  133. 8.3. Finding Quoted URLs in Full Text
  134. 8.4. Finding URLs with Parentheses in Full Text
  135. 8.5. Turn URLs into Links
  136. 8.6. Validating URNs
  137. 8.7. Validating Generic URLs
  138. 8.8. Extracting the Scheme from a URL
  139. 8.9. Extracting the User from a URL
  140. 8.10. Extracting the Host from a URL
  141. 8.11. Extracting the Port from a URL
  142. 8.12. Extracting the Path from a URL
  143. 8.13. Extracting the Query from a URL
  144. 8.14. Extracting the Fragment from a URL
  145. 8.15. Validating Domain Names
  146. 8.16. Matching IPv4 Addresses
  147. 8.17. Matching IPv6 Addresses
  148. 8.18. Validate Windows Paths
  149. 8.19. Split Windows Paths into Their Parts
  150. 8.20. Extract the Drive Letter from a Windows Path
  151. 8.21. Extract the Server and Share from a UNC Path
  152. 8.22. Extract the Folder from a Windows Path
  153. 8.23. Extract the Filename from a Windows Path
  154. 8.24. Extract the File Extension from a Windows Path
  155. 8.25. Strip Invalid Characters from Filenames
  156. 9. Markup and Data Formats
  157. Processing Markup and Data Formats with Regular Expressions
  158. 9.1. Find XML-Style Tags
  159. 9.2. Replace Tags with
  160. 9.3. Remove All XML-Style Tags Except and
  161. 9.4. Match XML Names
  162. 9.5. Convert Plain Text to HTML by Adding

    and
    Tags

  163. 9.6. Decode XML Entities
  164. 9.7. Find a Specific Attribute in XML-Style Tags
  165. 9.8. Add a cellspacing Attribute to Tags That Do Not Already Include It
  166. 9.9. Remove XML-Style Comments
  167. 9.10. Find Words Within XML-Style Comments
  168. 9.11. Change the Delimiter Used in CSV Files
  169. 9.12. Extract CSV Fields from a Specific Column
  170. 9.13. Match INI Section Headers
  171. 9.14. Match INI Section Blocks
  172. 9.15. Match INI Name-Value Pairs
  173. Index
  174. Index
  175. Index
  176. Index
  177. Index
  178. Index
  179. Index
  180. Index
  181. Index
  182. Index
  183. Index
  184. Index
  185. Index
  186. Index
  187. Index
  188. Index
  189. Index
  190. Index
  191. Index
  192. Index
  193. Index
  194. Index
  195. Index
  196. Index
  197. Index
  198. Index
  199. About the Authors
  200. Colophon
  201. Copyright

    202. 4.5. Validate Traditional Date Formats, Excluding Invalid Dates

    Problem

    You want to validate dates in the traditional formats mm/dd/yy, mm/dd/yyyy, dd/mm/yy, and dd/mm/yyyy, as shown in Recipe 4.4. But this time, you also want to weed out invalid dates, such as February 31st.

    Solution

    C#

    The first solution requires the month to be specified before the day. The regular expression works with a variety of flavors:

    ^(?<month>[0-3]?[0-9])/(?<day>[0-3]?[0-9])/(?<year>(?:[0-9]{2})?[0-9]{2})$
    Regex options: None
    Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10

    This is the complete solution implemented in C#:

    DateTime foundDate;
Match matchResult = Regex.Match(SubjectString,
    "^(?<month>[0-3]?[0-9])/(?<day>[0-3]?[0-9])/" +
    "(?<year>(?:[0-9]{2})?[0-9]{2})$");
if (matchResult.Success) {
    int year = int.Parse(matchResult.Groups["year"].Value);
    if (year < 50) year += 2000;
    else if (year < 100) year += 1900;
    try {
        foundDate = new DateTime(year,
            int.Parse(matchResult.Groups["month"].Value),
            int.Parse(matchResult.Groups["day"].Value));
    } catch {
        // Invalid date
    }
}

    The second solution requires the day to be specified before the month. The only difference is that we’ve swapped the names of the capturing groups in the regular expression.

    ^(?<day>[0-3]?[0-9])/(?<month>[0-3]?[0-9])/(?<year>(?:[0-9]{2})?[0-9]{2})$
    Regex options: None
    Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10

    The C# code is unchanged, except for the regular expression:

    DateTime foundDate;
Match matchResult = Regex.Match(SubjectString,
    "^(?<day>[0-3]?[0-9])/(?<month>[0-3]?[0-9])/" +
    "(?<year>(?:[0-9]{2})?[0-9]{2})$");
if (matchResult.Success) {
    int year = int.Parse(matchResult.Groups["year"].Value);
    if (year < 50) year += 2000;
    else if (year < 100) year += 1900;
    try {
        foundDate = new DateTime(year,
            int.Parse(matchResult.Groups["month"].Value),
            int.Parse(matchResult.Groups["day"].Value));
    } catch {
        // Invalid date
    }
}

Perl

The first solution requires the month to be specified before the day. The regular expression works with all flavors covered in this book.

^([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

This is the complete solution implemented in Perl:

@daysinmonth = (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);
$validdate = 0;
if ($subject =~ m!^([0-3]?[0-9])/([0-3]?[0-9])/((?:[0-9]{2})?[0-9]{2})$!) 
{
    $month = $1;
    $day = $2;
    $year = $3;
    $year += 2000 if $year < 50;
    $year += 1900 if $year < 100;
    if ($month == 2 && $year % 4 == 0 && ($year % 100 != 0 ||
                                          $year % 400 == 0)) {
    	$validdate = 1 if $day >= 1 && $day <= 29;
    } elsif ($month >= 1 && $month <= 12) {
        $validdate = 1 if $day >= 1 && $day <= $daysinmonth[$month-1];
    }
}

The second solution requires the day to be specified before the month. The regular expression is exactly the same. The Perl code swaps the meaning of the first two capturing groups.

@daysinmonth = (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);
$validdate = 0;
if ($subject =~ m!^([0-3]?[0-9])/([0-3]?[0-9])/((?:[0-9]{2})?[0-9]{2})$!) 
{
    $day = $1;
    $month = $2;
    $year = $3;
    $year += 2000 if $year < 50;
    $year += 1900 if $year < 100;
    if ($month == 2 && $year % 4 == 0 && ($year % 100 != 0 ||
                                          $year % 400 == 0)) {
    	$validdate = 1 if $day >= 1 && $day <= 29;
    } elsif ($month >= 1 && $month <= 12) {
        $validdate = 1 if $day >= 1 && $day <= $daysinmonth[$month-1];
    }
}

Pure regular expression

You can solve this problem with one regular expression without procedural code, if that is all you can use in your application.

Month before day:

^(?:
  # February (29 days every year)
  (?<month>0?2)/(?<day>[12][0-9]|0?[1-9])
|
  # 30-day months
  (?<month>0?[469]|11)/(?<day>30|[12][0-9]|0?[1-9])
|
  # 31-day months
  (?<month>0?[13578]|1[02])/(?<day>3[01]|[12][0-9]|0?[1-9])
)
# Year
/(?<year>(?:[0-9]{2})?[0-9]{2})$
Regex options: Free-spacing
Regex flavors: .NET, Perl 5.10, Ruby 1.9
^(?:
  # February (29 days every year)
  (0?2)/([12][0-9]|0?[1-9])
|
  # 30-day months
  (0?[469]|11)/(30|[12][0-9]|0?[1-9])
|
  # 31-day months
  (0?[13578]|1[02])/(3[01]|[12][0-9]|0?[1-9])
)
# Year
/((?:[0-9]{2})?[0-9]{2})$
Regex options: Free-spacing
Regex flavors: .NET, Java, XRegExp, PCRE, Perl, Python, Ruby
^(?:(0?2)/([12][0-9]|0?[1-9])|(0?[469]|11)/(30|[12][0-9]|0?[1-9])|↵
(0?[13578]|1[02])/(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

Day before month:

^(?:
  # February (29 days every year)
  (?<day>[12][0-9]|0?[1-9])/(?<month>0?2)
|
  # 30-day months
  (?<day>30|[12][0-9]|0?[1-9])/(?<month>0?[469]|11)
|
  # 31-day months
  (?<day>3[01]|[12][0-9]|0?[1-9])/(?<month>0?[13578]|1[02])
)
# Year
/(?<year>(?:[0-9]{2})?[0-9]{2})$
Regex options: Free-spacing
Regex flavors: .NET, Perl 5.10, Ruby 1.9
^(?:
  # February (29 days every year)
  ([12][0-9]|0?[1-9])/(0?2)
|
  # 30-day months
  (30|[12][0-9]|0?[1-9])/([469]|11)
|
  # 31-day months
  (3[01]|[12][0-9]|0?[1-9])/(0?[13578]|1[02])
)
# Year
/((?:[0-9]{2})?[0-9]{2})$
Regex options: Free-spacing
Regex flavors: .NET, Java, XRegExp, PCRE, Perl, Python, Ruby
^(?:([12][0-9]|0?[1-9])/(0?2)|(30|[12][0-9]|0?[1-9])/([469]|11)|↵
(3[01]|[12][0-9]|0?[1-9])/(0?[13578]|1[02]))/((?:[0-9]{2})?[0-9]{2})$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Discussion

Regex with procedural code

There are essentially two ways to accurately validate dates with a regular expression. One method is to use a simple regex that merely captures groups of numbers that look like a month/day/year combination, and then use procedural code to check whether the date is correct.

The main benefit of this method is that you can easily add additional restrictions, such as limiting dates to certain periods. Many programming languages provide specific support for dealing with dates. The C# solution uses .NET’s DateTime structure to check whether the date is valid and return the date in a useful format, all in one step.

We used the first regex from Recipe 4.4 that allows any number between 0 and 39 for the day and month. That makes it easy to change the format from mm/dd/yy to dd/mm/yy by changing which capturing group is treated as the month. When we’re using named capture, that means changing the names of the capturing groups in the regular expression. When we’re using numbered capture, that means changing the references to the numbered groups in the procedural code.

Pure regular expression

The other method is to do everything with a regular expression. We can use the same technique of spelling out the alternatives as we did for the more final solutions presented in Recipe 4.4. The solution is manageable, if we take the liberty of treating every year as a leap year, allowing the regex to match February 29th regardless of the year. Allowing February 29th only on leap years would require us to spell out all the years that are leap years, and all the years that aren’t.

The problem with using a single regular expression is that it no longer neatly captures the day and month in a single capturing group. We now have three capturing groups for the month, and three for the day. When the regex matches a date, only three of the seven groups in the regex will actually capture something. If the month is February, groups 1 and 2 capture the month and day. If the month has 30 days, groups 3 and 4 return the month and day. If the month has 31 days, groups 5 and 6 take action. Group 7 always captures the year.

Perl 5.10, Ruby 1.9, and .NET help us in this situation. Their regex flavors allow multiple named capturing groups to share the same name. See the section Groups with the same name in Recipe 2.11 for details. We take advantage of this by using the same names “month” and “day” in each of the alternatives. When the regex finds a match, we can retrieve the text matched by the groups “month” and “day” without worrying about how many days the month has.

For the other regex flavors, we use numbered capturing groups. When a match is found, three different groups have to be checked to extract the day, and three other groups to extract the month.

The pure regex solution is interesting only in situations where one regex is all you can use, such as when you’re using an application that offers one box to type in a regex. When programming, make things easier with a bit of extra code. This will be particularly helpful if you want to add extra checks on the date later.

Variations

To show how complicated the pure regex solution gets as you add more requirements, here’s a pure regex solution that matches any date between 2 May 2007 and 29 August 2008 in d/m/yy or dd/mm/yyyy format:

# 2 May 2007 till 29 August 2008
^(?:
  # 2 May 2007 till 31 December 2007
  (?:
    # 2 May till 31 May
    (?<day>3[01]|[12][0-9]|0?[2-9])/(?<month>0?5)/(?<year>2007)
  |
    # 1 June till 31 December
    (?:
      # 30-day months
      (?<day>30|[12][0-9]|0?[1-9])/(?<month>0?[69]|11)
    |
      # 31-day months
      (?<day>3[01]|[12][0-9]|0?[1-9])/(?<month>0?[78]|1[02])
    )
    /(?<year>2007)
  )
|
  # 1 January 2008 till 29 August 2008
  (?:
    # 1 August till 29 August
    (?<day>[12][0-9]|0?[1-9])/(?<month>0?8)/(?<year>2008)
  |
    # 1 Janary till 30 June
    (?:
      # February
      (?<day>[12][0-9]|0?[1-9])/(?<month>0?2)
    |
      # 30-day months
      (?<day>30|[12][0-9]|0?[1-9])/(?<month>0?[46])
    |
      # 31-day months
      (?<day>3[01]|[12][0-9]|0?[1-9])/(?<month>0?[1357])
    )
    /(?<year>2008)
  )
)$
Regex options: Free-spacing
Regex flavors: .NET, Perl 5.10, Ruby 1.9

See Also

This chapter has several other recipes for matching dates and times. Recipe 4.5 shows how to validate traditional date formats more simply, giving up some accuracy. 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.

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 2.11 explains named capturing groups. Recipe 2.12 explains repetition.