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. 4.21. European VAT Numbers

    Problem

    You’re given the job of implementing an online order form for a business in the European Union.

    European tax laws stipulate that when a VAT-registered business (your customer) located in one EU country purchases from a vendor (your company) in another EU country, the vendor must not charge VAT (Value-Added Tax). If the buyer is not VAT-registered, the vendor must charge VAT and remit the VAT to the local tax office. The vendor must use the VAT registration number of the buyer as proof to the tax office that no VAT is due. This means that for the vendor, it is very important to validate the buyer’s VAT number before proceeding with the tax-exempt sale.

    The most common cause of invalid VAT numbers are simple typing mistakes by the customer. To make the ordering process faster and friendlier, you should use a regular expression to validate the VAT number immediately while the customer fills out your online order form. You can do this with some client-side JavaScript or in the CGI script on your web server that receives the order form. If the number does not match the regular expression, the customer can correct the typo right away.

    Solution

    To keep the implementation simple, this solution is split into two parts. First we strip out spaces and punctuation. Then we validate what remains.

    Strip whitespace and punctuation

    Retrieve the VAT number entered by the customer and store it into a variable. Before performing the check for a valid number, replace all matches of this regular expression with a blank replacement text:

    [-.●]
    Regex options: None
    Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

    Recipe 3.14 shows you how to perform this initial replacement. We’ve assumed that the customer wouldn’t enter any punctuation except hyphens, dots, and spaces. Any other extraneous characters will be caught by the upcoming check.

Validate the number

With whitespace and punctuation stripped, this regular expression checks whether the VAT number is valid for any of the 27 EU countries:

^(
(AT)?U[0-9]{8} |                              # Austria
(BE)?0[0-9]{9} |                              # Belgium
(BG)?[0-9]{9,10} |                            # Bulgaria
(CY)?[0-9]{8}L |                              # Cyprus
(CZ)?[0-9]{8,10} |                            # Czech Republic
(DE)?[0-9]{9} |                               # Germany
(DK)?[0-9]{8} |                               # Denmark
(EE)?[0-9]{9} |                               # Estonia
(EL|GR)?[0-9]{9} |                            # Greece
(ES)?[0-9A-Z][0-9]{7}[0-9A-Z] |               # Spain
(FI)?[0-9]{8} |                               # Finland
(FR)?[0-9A-Z]{2}[0-9]{9} |                    # France
(GB)?([0-9]{9}([0-9]{3})?|[A-Z]{2}[0-9]{3}) | # United Kingdom
(HU)?[0-9]{8} |                               # Hungary
(IE)?[0-9]S[0-9]{5}L |                        # Ireland
(IT)?[0-9]{11} |                              # Italy
(LT)?([0-9]{9}|[0-9]{12}) |                   # Lithuania
(LU)?[0-9]{8} |                               # Luxembourg
(LV)?[0-9]{11} |                              # Latvia
(MT)?[0-9]{8} |                               # Malta
(NL)?[0-9]{9}B[0-9]{2} |                      # Netherlands
(PL)?[0-9]{10} |                              # Poland
(PT)?[0-9]{9} |                               # Portugal
(RO)?[0-9]{2,10} |                            # Romania
(SE)?[0-9]{12} |                              # Sweden
(SI)?[0-9]{8} |                               # Slovenia
(SK)?[0-9]{10}                                # Slovakia
)$
Regex options: Free-spacing, case insensitive
Regex flavors: .NET, Java, XRegExp, PCRE, Perl, Python, Ruby

The above regular expression uses free-spacing mode to make it easy to edit later. Every now and then, new countries join the European Union, and member countries change their rules for VAT numbers. Unfortunately, JavaScript does not support free-spacing. In this case, you’re stuck putting everything on one line:

^((AT)?U[0-9]{8}|(BE)?0[0-9]{9}|(BG)?[0-9]{9,10}|(CY)?[0-9]{8}L|↵
(CZ)?[0-9]{8,10}|(DE)?[0-9]{9}|(DK)?[0-9]{8}|(EE)?[0-9]{9}|↵
(EL|GR)?[0-9]{9}|(ES)?[0-9A-Z][0-9]{7}[0-9A-Z]|(FI)?[0-9]{8}|↵
(FR)?[0-9A-Z]{2}[0-9]{9}|(GB)?([0-9]{9}([0-9]{3})?|[A-Z]{2}[0-9]{3})|↵
(HU)?[0-9]{8}|(IE)?[0-9]S[0-9]{5}L|(IT)?[0-9]{11}|↵
(LT)?([0-9]{9}|[0-9]{12})|(LU)?[0-9]{8}|(LV)?[0-9]{11}|(MT)?[0-9]{8}|↵
(NL)?[0-9]{9}B[0-9]{2}|(PL)?[0-9]{10}|(PT)?[0-9]{9}|(RO)?[0-9]{2,10}|↵
(SE)?[0-9]{12}|(SI)?[0-9]{8}|(SK)?[0-9]{10})$
Regex options: Case insensitive
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Follow Recipe 3.6 to add this regular expression to your order form.

Discussion

Strip whitespace and punctuation

To make VAT numbers easier to read for humans, people often type them in with extra punctuation to split the digits into groups. For instance, a German customer might enter his VAT number DE123456789 as DE 123.456.789.

A single regular expression that matches VAT numbers from 27 countries in any possible notation is an impossible job. Since the punctuation is only for readability, it is much easier to first strip all the punctuation, then validate the resulting bare VAT number.

The regular expression ‹[-.●]› matches a character that is a hyphen, dot, or space. Replacing all matches of this regular expression with nothing effectively deletes the punctuation characters commonly used in VAT numbers.

Tip

VAT numbers consist only of letters and digits. Instead of using ‹[-.●]› to remove only common punctuation, you could use ‹[^A-Z0-9]› to strip out all invalid characters.

Validate the number

The two regular expressions for validating the number are identical. The only difference is that the first one uses the free-spacing syntax to make the regular expression more readable, and to indicate the countries. JavaScript does not support free-spacing unless you use the XRegExp library. The other flavors give you the choice.

The regex uses alternation to accommodate the VAT numbers of all 27 EU countries. The essential formats are shown in Table 4-3.

Table 4-3. EU VAT number formats

Country

VAT number format

Austria

U99999999

Belgium

0999999999

Bulgaria

999999999 or 9999999999

Cyprus

99999999L

Czech Republic

99999999, 999999999, or 9999999999

Germany

999999999

Denmark

99999999

Estonia

999999999

Greece

999999999

Spain

X9999999X

Finland

99999999

France

XX999999999

United Kingdom

999999999, 999999999999, or XX999

Hungary

99999999

Ireland

9S99999L

Italy

99999999999

Lithuania

999999999 or 99999999999

Luxembourg

99999999

Latvia

99999999999

Malta

99999999

Netherlands

999999999B99

Poland

999999999

Portugal

999999999

Romania

99, 999, 9999, 99999, 999999, 9999999, 99999999, 999999999, or 9999999999

Sweden

99999999999

Slovenia

99999999

Slovakia

999999999

Strictly speaking, the two-letter country code is part of the VAT number. However, people often omit it, since the billing address already indicates the country. The regular expression will accept VAT numbers with and without the country code. If you want the country code to be mandatory, remove all the question marks from the regular expression. If you do, mention that you require the country code in the error message that tells the user the VAT number is invalid.

If you accept orders only from certain countries, you can leave out the countries that don’t appear in the country selection on your order form. When you delete an alternative, make sure to also delete the ‹|› operator that separates the alternative from the next or previous one. If you don’t, you end up with ‹||› in your regular expression. ‹||› inserts an alternative that matches the empty string, which means your order form will accept the omission of a VAT number as a valid VAT number.

The 27 alternatives are grouped together. The group is placed between a caret and a dollar sign, which anchor the regular expression to the beginning and ending of the string you’re validating. The whole input must validate as a VAT number.

If you’re searching for VAT numbers in a larger body of text, replace the anchors with ‹\b› word boundaries.

Variations

The benefit of using one regular expression to check for all 27 countries is that you only need to add one regex validation to your order form. You could enhance your order form by using 27 separate regular expressions. First, check the country that the customer specified in the billing address. Then, look up the appropriate regular expression according to the country in Table 4-4.

Table 4-4. EU VAT number regular expressions

Country

VAT number regular expression

Austria

‹^(AT)?U[0-9]{8}$›

Belgium

‹^(BE)?0[0-9]{9}$›

Bulgaria

‹^(BG)?[0-9]{9,10}$›

Cyprus

‹^(CY)?[0-9]{8}L$›

Czech Republic

‹^(CZ)?[0-9]{8,10}$›

Germany

‹^(DE)?[0-9]{9}$›

Denmark

‹^(DK)?[0-9]{8}$›

Estonia

‹^(EE)?[0-9]{9}$›

Greece

‹^(EL|GR)?[0-9]{9}$›

Spain

‹^(ES)?[0-9A-Z][0-9]{7}[0-9A-Z]$›

Finland

‹^(FI)?[0-9]{8}$›

France

‹^(FR)?[0-9A-Z]{2}[0-9]{9}$›

United Kingdom

‹^(GB)?([0-9]{9}([0-9]{3})?|[A-Z]{2}[0-9]{3})$›

Hungary

‹^(HU)?[0-9]{8}$›

Ireland

‹^(IE)?[0-9]S[0-9]{5}L$›

Italy

‹^(IT)?[0-9]{11}$›

Lithuania

‹^(LT)?([0-9]{9}|[0-9]{12})$›

Luxembourg

‹^(LU)?[0-9]{8}$›

Latvia

‹^(LV)?[0-9]{11}$›

Malta

‹^(MT)?[0-9]{8}$›

Netherlands

‹^(NL)?[0-9]{9}B[0-9]{2}$›

Poland

‹^(PL)?[0-9]{10}$›

Portugal

‹^(PT)?[0-9]{9}$›

Romania

‹^(RO)?[0-9]{2,10}$›

Sweden

‹^(SE)?[0-9]{12}$›

Slovenia

‹^(SI)?[0-9]{8}$›

Slovakia

‹^(SK)?[0-9]{10}$›

Implement Recipe 3.6 to validate the VAT number against the selected regular expression. That will tell you if the number is valid for the country the customer claims to reside in.

The main benefit of using the separate regular expressions is that you can force the VAT number to start with the correct country code, without asking the customer to type it in. When the regular expression matches the provided number, check the contents of the first capturing group. Recipe 3.9 explains how to do this. If the first capturing group is empty, the customer did not type the country code at the start of the VAT number. You can then add the country code before storing the validated number in your order database.

Greek VAT numbers allow two country codes. EL is traditionally used for Greek VAT numbers, but GR is the ISO country code for Greece.

See Also

The regular expression merely checks if the number looks like a valid VAT number. This is enough to weed out honest mistakes. A regular expression obviously cannot check whether the VAT number is assigned to the business placing the order. The European Union provides a web page at http://ec.europa.eu/taxation_customs/vies/vieshome.do where you can check which business a particular VAT number belongs to, if any.

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 2.12 explains repetition.