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. 2.4. Match Any Character

    This recipe explains the ins and outs of the dot metacharacter.

    Problem

    Match a quoted character. Provide one solution that allows any single character, except a line break, between the quotes. Provide another that truly allows any character, including line breaks.

    Solution

    Any character except line breaks

    '.'
    Regex options: None (the “dot matches line breaks” option must not be set)
    Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Any character including line breaks

'.'
Regex options: Dot matches line breaks
Regex flavors: .NET, Java, PCRE, Perl, Python, Ruby
'[\s\S]'
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Discussion

Any character except line breaks

The dot is one of the oldest and simplest regular expression features. Its meaning has always been to match any single character.

There is, however, some confusion as to what any character truly means. The oldest tools for working with regular expressions processed files line by line, so there was never an opportunity for the subject text to include a line break. The programming languages discussed in this book process the subject text as a whole, no matter how many line breaks you put into it. If you want true line-by-line processing, you have to write a bit of code that splits the subject into an array of lines and applies the regex to each line in the array. Recipe 3.21 in the next chapter shows how to do this.

Larry Wall, the developer of Perl, wanted Perl to retain the traditional behavior of line-based tools, in which the dot never matched a line break. All the other flavors discussed in this book followed suit. ‹.› thus matches any single character except line break characters.

Any character including line breaks

If you do want to allow your regular expression to span multiple lines, turn on the “dot matches line breaks” option. This option masquerades under different names. Perl and many others confusingly call it “single line” mode, whereas Java calls it “dot all” mode. Recipe 3.4 in the next chapter has all the details. Whatever the name of this option in your favorite programming language is, think of it as “dot matches line breaks” mode. That’s all the option does.

An alternative solution is needed for JavaScript, which doesn’t have a “dot matches line breaks” option. As Recipe 2.3 explains, ‹\s› matches any whitespace character, whereas ‹\S› matches any character that is not matched by ‹\s›. Combining these into ‹[\s\S]› results in a character class that includes all characters, including line breaks. ‹[\d\D]› and ‹[\w\W]› have the same effect.

Dot abuse

The dot is the most abused regular expression feature. ‹\d\d.\d\d.\d\d› is not a good way to match a date. It does match 05/16/08 just fine, but it also matches 99/99/99. Worse, it matches 12345678.

A proper regex for matching only valid dates is a subject for a later chapter (see Recipe 4.5). But replacing the dot with a more appropriate character class is very easy. ‹\d\d[/.\-]\d\d[/.\-]\d\d› allows a forward slash, dot, or hyphen to be used as the date separator. This regex still matches 99/99/99, but not 12345678.

Tip

It’s just a coincidence that the previous example includes a dot inside the character classes. Inside a character class, the dot is just a literal character. It’s worth including in this particular regular expression because in some countries, such as Germany, the dot is used as a date separator.

Use the dot only when you really want to allow any character. Use a character class or negated character class in any other situation.

Variations

Here’s how to match any quoted character, including line breaks, with the help of an inline mode modifier:

(?s)'.'
Regex options: None
Regex flavors: .NET, Java, XRegExp, PCRE, Perl, Python
(?m)'.'
Regex options: None
Regex flavors: Ruby

If you cannot turn on “dot matches line breaks” mode outside the regular expression, you can place a mode modifier at the start of the regular expression. We explain the concept of mode modifiers, and JavaScript’s lack of support for them, in the subsection Case-insensitive matching in Recipe 2.1.

‹(?s)› is the mode modifier for “dot matches line breaks” mode in .NET, Java, XRegExp, PCRE, Perl, and Python. The s stands for “single line” mode, which is Perl’s confusing name for “dot matches line breaks.”

The terminology is so confusing that the developer of Ruby’s regex engine copied it wrongly. Ruby uses ‹(?m)› to turn on “dot matches line breaks” mode. Other than the different letter, the functionality is exactly the same. The new engine in Ruby 1.9 continues to use ‹(?m)› for “dot matches line breaks.” Perl’s very different meaning for ‹(?m)› is explained in Recipe 2.5.

See Also

In many cases, you don’t want to match truly any character, but rather any character except a select few. Recipe 2.3 explains how to do that.

Recipe 3.4 explains how to set options such as “dot matches line breaks” in your source code.

When working with Unicode text, you may prefer to use ‹\X› to match a Unicode grapheme instead of the dot which matches a Unicode code point. Recipe 2.7 explains this in detail.