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. 4.7. Validate ISO 8601 Dates and Times

    Problem

    You want to match dates and/or times in the official ISO 8601 format, which is the basis for many standardized date and time formats. For example, in XML Schema, the built-in date, time, and dateTime types are all based on ISO 8601.

    Solution

    The ISO 8601 standard defines a wide range of date and time formats. Most applications that use ISO 8601 only use a subset of it. These solutions match the most commonly used ISO 8601 date and time formats. We’ve also added solutions for XML Schema, which is one particular implementation of ISO 8601.

    Dates

    The following matches a calendar month (e.g., 2008-08). The hyphen is required:

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

    Named capture makes the regular expression and any code that may reference the capturing groups easier to read:

    ^(?<year>[0-9]{4})-(?<month>1[0-2]|0[1-9])$
    Regex options: None
    Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9

    Python uses a different syntax for named capture, adding a P. For brevity, we only show one solution using the Python syntax. All the other solutions using .NET-style named capture can be easily adapted to Python-style named capture in the same way.

    ^(?P<year>[0-9]{4})-(?P<month>1[0-2]|0[1-9])$
    Regex options: None
    Regex flavors: PCRE, Python

    ISO 8601 allows hyphens to be omitted from calendar dates, making both 2010-08-20 and 20100820 valid representations of the same date. The following regex accounts for this, but also allows for invalid formats like YYYY-MMDD and YYYYMM-DD.

    ^([0-9]{4})-?(1[0-2]|0[1-9])-?(3[01]|0[1-9]|[12][0-9])$
    Regex options: None
    Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
    ^(?<year>[0-9]{4})-?(?<month>1[0-2]|0[1-9])-?↵
(?<day>3[01]|0[1-9]|[12][0-9])$
    Regex options: None
    Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9

    Calendar date, such as 2008-08-30 or 20080830. The hyphens are optional. This regex uses a capturing group and a backreference to match YYYY-MM-DD or YYYYMMDD, but not YYYY-MMDD or YYYYMM-DD.

    ^([0-9]{4})(-?)(1[0-2]|0[1-9])\2(3[01]|0[1-9]|[12][0-9])$
    Regex options: None
    Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
    ^(?<year>[0-9]{4})(?<hyphen>-?)(?<month>1[0-2]|0[1-9])↵
\k<hyphen>(?<day>3[01]|0[1-9]|[12][0-9])$
    Regex options: None
    Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9

    Python also uses a different syntax for named backreferences:

    ^(?P<year>[0-9]{4})(?P<hyphen>-?)(?P<month>1[0-2]|0[1-9])↵
(?P=hyphen)(?<day>3[01]|0[1-9]|[12][0-9])$
    Regex options: None
    Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9

    Ordinal date (e.g., 2008-243). The hyphen is optional:

    ^([0-9]{4})-?(36[0-6]|3[0-5][0-9]|[12][0-9]{2}|0[1-9][0-9]|00[1-9])$
    Regex options: None
    Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
    ^(?<year>[0-9]{4})-?↵
(?<day>36[0-6]|3[0-5][0-9]|[12][0-9]{2}|0[1-9][0-9]|00[1-9])$
    Regex options: None
    Regex flavors: .NET, Java 7, PCRE 7, Perl 5.10, Ruby 1.9

Weeks

Week of the year (e.g., 2008-W35). The hyphen is optional:

^([0-9]{4})-?W(5[0-3]|[1-4][0-9]|0[1-9])$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
^(?<year>[0-9]{4})-?W(?<week>5[0-3]|[1-4][0-9]|0[1-9])$
Regex options: None
Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9

Week date (e.g., 2008-W35-6). The hyphens are optional.

^([0-9]{4})-?W(5[0-3]|[1-4][0-9]|0[1-9])-?([1-7])$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
^(?<year>[0-9]{4})-?W(?<week>5[0-3]|[1-4][0-9]|0[1-9])-?(?<day>[1-7])$
Regex options: None
Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9

Times

Hours and minutes (e.g., 17:21). The colon is optional:

^(2[0-3]|[01][0-9]):?([0-5][0-9])$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
^(?<hour>2[0-3]|[01][0-9]):?(?<minute>[0-5][0-9])$
Regex options: None
Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9

Hours, minutes, and seconds (e.g., 17:21:59). The colons are optional:

^(2[0-3]|[01][0-9]):?([0-5][0-9]):?([0-5][0-9])$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
^(?<hour>2[0-3]|[01][0-9]):?(?<minute>[0-5][0-9]):?↵
(?<second>[0-5][0-9])$
Regex options: None
Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9

Time zone designator (e.g., Z, +07 or +07:00). The colons and the minutes are optional:

^(Z|[+-](?:2[0-3]|[01][0-9])(?::?(?:[0-5][0-9]))?)$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Hours, minutes, and seconds with time zone designator (e.g., 17:21:59+07:00). All the colons are optional. The minutes in the time zone designator are also optional:

^(2[0-3]|[01][0-9]):?([0-5][0-9]):?([0-5][0-9])↵
(Z|[+-](?:2[0-3]|[01][0-9])(?::?(?:[0-5][0-9]))?)$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
^(?<hour>2[0-3]|[01][0-9]):?(?<minute>[0-5][0-9]):?(?<second>[0-5][0-9])↵
(?<timezone>Z|[+-](?:2[0-3]|[01][0-9])(?::?(?:[0-5][0-9]))?)$
Regex options: None
Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9

Date and time

Calendar date with hours, minutes, and seconds (e.g., 2008-08-30 17:21:59 or 20080830 172159). A space is required between the date and the time. The hyphens and colons are optional. This regex matches dates and times that specify some hyphens or colons but omit others. This does not follow ISO 8601.

^([0-9]{4})-?(1[0-2]|0[1-9])-?(3[01]|0[1-9]|[12][0-9])↵
●(2[0-3]|[01][0-9]):?([0-5][0-9]):?([0-5][0-9])$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
^(?<year>[0-9]{4})-?(?<month>1[0-2]|0[1-9])-?↵
(?<day>3[01]|0[1-9]|[12][0-9])●(?<hour>2[0-3]|[01][0-9])↵
:?(?<minute>[0-5][0-9]):?(?<second>[0-5][0-9])$
Regex options: None
Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9

A more complicated solution is needed if we want to match date and time values that specify either all of the hyphens and colons, or none of them. The cleanest solution is to use conditionals. But only some flavors support conditionals.

^([0-9]{4})(-)?(1[0-2]|0[1-9])(?(2)-)(3[01]|0[1-9]|[12][0-9])↵
●(2[0-3]|[01][0-9])(?(2):)([0-5][0-9])(?(2):)([0-5][0-9])$
Regex options: None
Regex flavors: .NET, PCRE, Perl, Python
^(?<year>[0-9]{4})(?<hyphen>-)?(?<month>1[0-2]|0[1-9])↵
(?(hyphen)-)(?<day>3[01]|0[1-9]|[12][0-9])●(?<hour>2[0-3]|[01][0-9])↵
(?(hyphen):)(?<minute>[0-5][0-9])(?(hyphen):)(?<second>[0-5][0-9])$
Regex options: None
Regex flavors: .NET, PCRE 7, Perl 5.10
^(?P<year>[0-9]{4})(?P<hyphen>-)?(?P<month>1[0-2]|0[1-9])↵
(?(hyphen)-)(?P<day>3[01]|0[1-9]|[12][0-9])●(?P<hour>2[0-3]|[01][0-9])↵
(?(hyphen):)(?P<minute>[0-5][0-9])(?(hyphen):)(?P<second>[0-5][0-9])$
Regex options: None
Regex flavors: PCRE, Perl 5.10, Python

If conditionals are not available, then we have to use alternation to spell out the alternatives with and without delimiters.

^(?:([0-9]{4})-?(1[0-2]|0[1-9])-?(3[01]|0[1-9]|[12][0-9])↵
●(2[0-3]|[01][0-9]):?([0-5][0-9]):?([0-5][0-9])|↵
([0-9]{4})(1[0-2]|0[1-9])(3[01]|0[1-9]|[12][0-9])↵
●(2[0-3]|[01][0-9])([0-5][0-9])([0-5][0-9]))$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

XML Schema dates and times

The date and time types defined in the XML Schema standard are based on the ISO 8601 standard. The date types allow negative years for years before the start of the calendar (B.C. years). It also allows for years with more than four digits, but not for years with fewer than four digits. Years with more than four digits must not have leading zeros. If you only want to allow years with four digits as in the preceding solutions, remove -?(?:[1-9][0-9]*)? from the following solutions.

Date, with optional time zone (e.g., 2008-08-30 or 2008-08-30+07:00). Hyphens are required. This is the XML Schema date type:

^(-?(?:[1-9][0-9]*)?[0-9]{4})-(1[0-2]|0[1-9])-(3[01]|0[1-9]|[12][0-9])↵
(Z|[+-](?:2[0-3]|[01][0-9]):[0-5][0-9])?$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
^(?<year>-?(?:[1-9][0-9]*)?[0-9]{4})-(?<month>1[0-2]|0[1-9])-↵
(?<day>3[01]|0[1-9]|[12][0-9])↵
(?<timezone>Z|[+-](?:2[0-3]|[01][0-9]):[0-5][0-9])?$
Regex options: None
Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9

Time, with optional fractional seconds and time zone (e.g., 01:45:36 or 01:45:36.123+07:00). There is no limit on the number of digits for the fractional seconds. This is the XML Schema time type:

^(2[0-3]|[01][0-9]):([0-5][0-9]):([0-5][0-9])(\.[0-9]+)?↵
(Z|[+-](?:2[0-3]|[01][0-9]):[0-5][0-9])?$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
^(?<hour>2[0-3]|[01][0-9]):(?<minute>[0-5][0-9]):(?<second>[0-5][0-9])↵
(?<frac>\.[0-9]+)?(?<timezone>Z|[+-](?:2[0-3]|[01][0-9]):[0-5][0-9])?$
Regex options: None
Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9

Date and time, with optional fractional seconds and time zone (e.g., 2008-08-30T01:45:36 or 2008-08-30T01:45:36.123Z). This is the XML Schema dateTime type:

^(-?(?:[1-9][0-9]*)?[0-9]{4})-(1[0-2]|0[1-9])-(3[01]|0[1-9]|[12][0-9])↵
T(2[0-3]|[01][0-9]):([0-5][0-9]):([0-5][0-9])(\.[0-9]+)?↵
(Z|[+-](?:2[0-3]|[01][0-9]):[0-5][0-9])?$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby
^(?<year>-?(?:[1-9][0-9]*)?[0-9]{4})-(?<month>1[0-2]|0[1-9])-↵
(?<day>3[01]|0[1-9]|[12][0-9])T(?<hour>2[0-3]|[01][0-9]):↵
(?<minute>[0-5][0-9]):(?<second>[0-5][0-9])(?<ms>\.[0-9]+)?↵
(?<timezone>Z|[+-](?:2[0-3]|[01][0-9]):[0-5][0-9])?$
Regex options: None
Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9

Discussion

ISO 8601 defines a wide range of date and time formats. The regular expressions presented here cover the most common formats, but most systems that use ISO 8601 only use a subset. For example, in XML Schema dates and times, the hyphens and colons are mandatory. To make hyphens and colons mandatory, simply remove the question marks after them. To disallow hyphens and colons, remove the hyphens and colons along with the question mark that follows them. Do watch out for the noncapturing groups, which use the ‹(?:⋯)› syntax. If a question mark and a colon follow an opening parenthesis, those three characters open a noncapturing group.

We put parentheses around all the number parts of the regexes. That makes it easy to retrieve the numbers for the years, months, days, hours, minutes, seconds, and time zones. Recipe 2.9 explains how parentheses create capturing groups. Recipe 3.9 explains how you can retrieve the text matched by those capturing groups in procedural code.

For most regexes, we also show an alternative using named capture. Some of these date and time formats may be unfamiliar to you or your fellow developers. Named capture makes the regex easier to understand. .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, and Ruby 1.9 support the ‹(?<name>⋯)› syntax used in the solutions in this recipe. All versions of PCRE and Python covered in this book support the alternative ‹(?P<name>⋯)› syntax, which adds a ‹P›. See Recipes 2.11 and 3.9 for details.

The number ranges in all the regexes are strict. For example, the calendar day is restricted between 01 and 31. You’ll never end up with day 32 or month 13. None of the regexes here attempts to exclude invalid day and month combinations, such as February 31st; Recipe 4.5 explains how you can deal with that.

The regular expressions, except those in the XML Schema subsection, make the individual hyphens and colons optional. This does not follow ISO 8601 exactly. For example, 1733:26 is not a valid ISO 8601 time, but will be accepted by the time regexes. Requiring all hyphens and colons to be present or omitted at the same time makes your regex quite a bit more complex.

If the delimiters are all the same, we can do this quite easily using a capturing group for the first delimiter and backreferences for the remaining delimiters. The “dates” subsection of the “Solution” section shows an example. For the first hyphen, we use ‹(-?)›, ‹(?<hyphen>-?)› or ‹(?P<hyphen>-?)› to match an optional hyphen and capture it into a named or numbered group. If the hyphen was omitted, the capturing group stores the zero-length string. The question mark that makes the hyphen optional must be inside the group. If we made the group itself optional, then backreferences to that group would always fail to match if the hyphen was not matched, as the group would not have participated in the match at all. For the remaining hyphens, we use ‹\2›, ‹\k<hyphen>›, or ‹(?P=hyphen)› to match the same text that was matched by the capturing group, which is either a hyphen or nothing at all, depending on whether the first hyphen was matched or not. When using numbered capture, make sure to use the correct number for the backreference.

If the delimiters are different, such as when matching a single string with both a date and a time, the solution is more complex. The “date and time” subsection shows an example. This time, we use ‹(-)?›, ‹(?<hyphen>-)?› or ‹(?P<hyphen>-)?› to match the hyphen. Now the question mark is outside the capturing group so that it will not participate in the match at all when the hyphen is omitted. This allows us to use the capturing group with a conditional. ‹(?(2)-)› matches a hyphen and ‹(?(2):)› matches a colon if the second capturing group participated in the match. The conditionals have no alternative, which means they will match nothing at all (but still succeed) when the second capturing group did not participate in the match. ‹(?(hyphen)-)› and ‹(?(hyphen):)› do the same using named capture.

Only some flavors support conditionals. If conditionals are not available, the only solution is to use alternation to spell out the two alternatives with and without delimiters. The disadvantage of this solution is that it results in two capturing groups for each part of the date and time. Only one of the two sets of capturing groups will participate in the match. Code that uses this regex will have to check both groups.

See Also

This chapter has several other recipes for matching dates and times. Recipes 4.4 and 4.5 show how to validate traditional date formats. Recipe 4.6 shows how to validate traditional time formats.

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 2.10 explains backreferences. Recipe 2.11 explains named capturing groups. Recipe 2.12 explains repetition. Recipe 2.17 explains conditionals.