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. 5.14. Escape Regular Expression Metacharacters

    Problem

    You want to use a literal string provided by a user or from some other source as part of a regular expression. However, you want to escape all regular expression metacharacters within the string before embedding it in your regex, to avoid any unintended consequences.

    Solution

    By adding a backslash before any characters that potentially have special meaning within a regular expression, you can safely use the resulting pattern to match a literal sequence of characters. Of the programming languages covered by this book, all except JavaScript have a built-in function or method to perform this task (listed in Table 5-3). However, for the sake of completeness, we’ll show how to pull this off using your own regex, even in the languages that have a ready-made solution.

    Built-in solutions

    Table 5-3 lists the built-in functions and methods designed to solve this problem.

    Table 5-3. Built-in solutions for escaping regular expression metacharacters

    Language

    Function

    C#, VB.NET

    Regex.Escape(str)

    Java

    Pattern.quote(str)

    XRegExp

    XRegExp.escape(str)

    Perl

    quotemeta(str)

    PHP

    preg_quote(str, [delimiter])

    Python

    re.escape(str)

    Ruby

    Regexp.escape(str)

Notably absent from the list is JavaScript (without XRegExp), which does not have a native function designed for this purpose.

Regular expression

Although it’s best to use a built-in solution if available, you can pull this off on your own by using the following regular expression along with the appropriate replacement string (shown next). Make sure to replace all matches, rather than only the first. Recipe 3.15 shows code for replacing matches with strings that contain backreferences. You’ll need a backreference here to bring back the matched special character along with a preceding backslash:

[[\]{}()*+?.\\|^$\-,&#\s]
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Replacement

Tip

The following replacement strings contain a literal backslash character. The strings are shown without the extra backslashes that may be needed to escape backslashes when using string literals in some programming languages. See Recipe 2.19 for more details about replacement text flavors.

\$&
Replacement text flavors: .NET, JavaScript
\$0
Replacement text flavors: .NET, XRegExp
\\$&
Replacement text flavor: Perl
\\$0
Replacement text flavors: Java, PHP
\\\0
Replacement text flavors: PHP, Ruby
\\\&
Replacement text flavor: Ruby
\\\g<0>
Replacement text flavor: Python

Example JavaScript function

Here’s an example of how you can put the regular expression and replacement string to use to create a static method called RegExp.escape() in JavaScript:

RegExp.escape = function(str) {
    return str.replace(/[[\]{}()*+?.\\|^$\-,&#\s]/g, "\\$&");
};

// Test it...
var str = "<Hello World.>";
var escapedStr = RegExp.escape(str);
alert(escapedStr == "<Hello\\ World\\.>"); // -> true

Discussion

This recipe’s regular expression puts all the regex metacharacters inside a single character class. Let’s take a look at each of those characters, and examine why they need to be escaped. Some are less obvious than others:

[ { ( )

‹[› creates a character class. ‹{› creates an interval quantifier and is also used with some other special constructs, such as Unicode properties. ‹(› and ‹)› are used for grouping, capturing, and other special constructs.

* + ?

These three characters are quantifiers that repeat their preceding element zero or more, one or more, or between zero and one time, respectively. The question mark is also used after an opening parenthesis to create special groupings and other constructs (the same is true for the asterisk in Perl 5.10 and PCRE 7).

. \ |

A dot matches any character within a line or string, a backslash makes a special character literal or a literal character special, and a vertical bar alternates between multiple options.

^ $

The caret and dollar symbols are anchors that match the start or end of a line or string. The caret can also negate a character class.

The remaining characters matched by the regular expression are only special in special circumstances. They’re included in the list to err on the side of caution.

]

A right square bracket ends a character class. Normally, this would not need to be escaped on its own, but doing so avoids unintentionally ending a character class when embedding text inside one. Keep in mind that if you do embed text inside a character class, the resulting regex will not match the embedded string, but rather any one of the characters in the embedded string.

-

A hyphen creates a range within a character class. It’s escaped here to avoid inadvertently creating ranges when embedding text in the middle of a character class.

}

A right curly bracket ends an interval quantifier or other special construct. Since most regular expression flavors treat curly brackets as literal characters if they do not form a valid quantifier, it’s possible to create a quantifier where there was none before when inserting literal text in a regex if you don’t escape both ends of curly brackets.

,

A comma is used inside an interval quantifier such as ‹{1,5}›. It’s possible (though a bit unlikely) to create a quantifier where there was none before when inserting literal text in a regex if you don’t escape commas.

&

The ampersand is included in the list because two ampersands in a row are used for character class intersection in Java (see Flavor-Specific Features). In other programming languages, it’s safe to remove the ampersand from the list of characters that need to be escaped, but it doesn’t hurt to keep it.

# and whitespace

The pound sign and whitespace (matched by ‹\s›) are metacharacters only if the free-spacing option is enabled. Again, it doesn’t hurt to escape them anyway.

As for the replacement text, one of five tokens («$&», «\&», «$0», «\0», or «\g<0>») is used to restore the matched character along with a preceding backslash. In Perl, $& is actually a variable, and using it with any regular expression imposes a global performance penalty on all regular expressions. If $& is used elsewhere in your Perl program, it’s OK to use it as much as you want because you’ve already paid the price. Otherwise, it’s probably better to wrap the entire regex in a capturing group, and use $1 instead of $& in the replacement.

Variations

As explained in Block escape, you can create a block escape sequence within a regex using ‹\Q⋯\E›. However, block escapes are only supported by Java, PCRE, and Perl, and even in those languages block escapes are not foolproof. For complete safety, you’d still need to escape any occurrence of \E within the string you plan to embed in your regex. In most cases it’s probably easier to just use the cross-language approach of escaping all regex metacharacters.

See Also

Recipe 2.1 discusses how to match literal characters and escape metacharacters. However, its list of characters that need to be escaped is shorter since it doesn’t concern itself with characters that may need to be escaped in free-spacing mode or when dropped into an arbitrary, longer pattern.

The example JavaScript solution in Recipe 5.2 creates a function that escapes any regular expression metacharacters within words to be searched for. It uses the shorter list of special characters from Recipe 2.1.

Techniques used in the regular expression and replacement text in this recipe are discussed in Chapter 2. Recipe 2.3 explains character classes. Recipe 2.20 explains how to insert the regex match into the replacement text.