5.13. Replace Repeated Whitespace with a Single Space
by Steven Levithan, Jan Goyvaerts
Regular Expressions Cookbook, 2nd Edition
- Regular Expressions Cookbook
- Preface
- 1. Introduction to Regular Expressions
- 2. Basic Regular Expression Skills
- 2.1. Match Literal Text
- 2.2. Match Nonprintable Characters
- 2.3. Match One of Many Characters
- 2.4. Match Any Character
- 2.5. Match Something at the Start and/or the End of a Line
- 2.6. Match Whole Words
- 2.7. Unicode Code Points, Categories, Blocks, and Scripts
- 2.8. Match One of Several Alternatives
- 2.9. Group and Capture Parts of the Match
- 2.10. Match Previously Matched Text Again
- 2.11. Capture and Name Parts of the Match
- 2.12. Repeat Part of the Regex a Certain Number of Times
- 2.13. Choose Minimal or Maximal Repetition
- 2.14. Eliminate Needless Backtracking
- 2.15. Prevent Runaway Repetition
- 2.16. Test for a Match Without Adding It to the Overall Match
- 2.17. Match One of Two Alternatives Based on a Condition
- 2.18. Add Comments to a Regular Expression
- 2.19. Insert Literal Text into the Replacement Text
- 2.20. Insert the Regex Match into the Replacement Text
- 2.21. Insert Part of the Regex Match into the Replacement Text
- 2.22. Insert Match Context into the Replacement Text
- 3. Programming with Regular Expressions
- Programming Languages and Regex Flavors
- 3.1. Literal Regular Expressions in Source Code
- 3.2. Import the Regular Expression Library
- 3.3. Create Regular Expression Objects
- 3.4. Set Regular Expression Options
- 3.5. Test If a Match Can Be Found Within a Subject String
- 3.6. Test Whether a Regex Matches the Subject String Entirely
- 3.7. Retrieve the Matched Text
- 3.8. Determine the Position and Length of the Match
- 3.9. Retrieve Part of the Matched Text
- 3.10. Retrieve a List of All Matches
- 3.11. Iterate over All Matches
- 3.12. Validate Matches in Procedural Code
- 3.13. Find a Match Within Another Match
- 3.14. Replace All Matches
- 3.15. Replace Matches Reusing Parts of the Match
- 3.16. Replace Matches with Replacements Generated in Code
- 3.17. Replace All Matches Within the Matches of Another Regex
- 3.18. Replace All Matches Between the Matches of Another Regex
- 3.19. Split a String
- 3.20. Split a String, Keeping the Regex Matches
- 3.21. Search Line by Line
- Construct a Parser
- 4. Validation and Formatting
- 4.1. Validate Email Addresses
- 4.2. Validate and Format North American Phone Numbers
- 4.3. Validate International Phone Numbers
- 4.4. Validate Traditional Date Formats
- 4.5. Validate Traditional Date Formats, Excluding Invalid Dates
- 4.6. Validate Traditional Time Formats
- 4.7. Validate ISO 8601 Dates and Times
- 4.8. Limit Input to Alphanumeric Characters
- 4.9. Limit the Length of Text
- 4.10. Limit the Number of Lines in Text
- 4.11. Validate Affirmative Responses
- 4.12. Validate Social Security Numbers
- 4.13. Validate ISBNs
- 4.14. Validate ZIP Codes
- 4.15. Validate Canadian Postal Codes
- 4.16. Validate U.K. Postcodes
- 4.17. Find Addresses with Post Office Boxes
- 4.18. Reformat Names From “FirstName LastName” to “LastName, FirstName”
- 4.19. Validate Password Complexity
- Problem
- Solution
- Length between 8 and 32 characters
- ASCII visible and space characters only
- One or more uppercase letters
- One or more lowercase letters
- One or more numbers
- One or more special characters
- Disallow three or more sequential identical characters
- Example JavaScript solution, basic
- Example JavaScript solution, with x out of y validation
- Example JavaScript solution, with password security ranking
- Discussion
- Variations
- See Also
- 4.20. Validate Credit Card Numbers
- 4.21. European VAT Numbers
- 5. Words, Lines, and Special Characters
- 5.1. Find a Specific Word
- 5.2. Find Any of Multiple Words
- 5.3. Find Similar Words
- 5.4. Find All Except a Specific Word
- 5.5. Find Any Word Not Followed by a Specific Word
- 5.6. Find Any Word Not Preceded by a Specific Word
- 5.7. Find Words Near Each Other
- 5.8. Find Repeated Words
- 5.9. Remove Duplicate Lines
- 5.10. Match Complete Lines That Contain a Word
- 5.11. Match Complete Lines That Do Not Contain a Word
- 5.12. Trim Leading and Trailing Whitespace
- 5.13. Replace Repeated Whitespace with a Single Space
- 5.14. Escape Regular Expression Metacharacters
- 6. Numbers
- 6.1. Integer Numbers
- 6.2. Hexadecimal Numbers
- 6.3. Binary Numbers
- 6.4. Octal Numbers
- 6.5. Decimal Numbers
- 6.6. Strip Leading Zeros
- 6.7. Numbers Within a Certain Range
- 6.8. Hexadecimal Numbers Within a Certain Range
- 6.9. Integer Numbers with Separators
- 6.10. Floating-Point Numbers
- 6.11. Numbers with Thousand Separators
- 6.12. Add Thousand Separators to Numbers
- 6.13. Roman Numerals
- 7. Source Code and Log Files
- 8. URLs, Paths, and Internet Addresses
- 8.1. Validating URLs
- 8.2. Finding URLs Within Full Text
- 8.3. Finding Quoted URLs in Full Text
- 8.4. Finding URLs with Parentheses in Full Text
- 8.5. Turn URLs into Links
- 8.6. Validating URNs
- 8.7. Validating Generic URLs
- 8.8. Extracting the Scheme from a URL
- 8.9. Extracting the User from a URL
- 8.10. Extracting the Host from a URL
- 8.11. Extracting the Port from a URL
- 8.12. Extracting the Path from a URL
- 8.13. Extracting the Query from a URL
- 8.14. Extracting the Fragment from a URL
- 8.15. Validating Domain Names
- 8.16. Matching IPv4 Addresses
- 8.17. Matching IPv6 Addresses
- 8.18. Validate Windows Paths
- 8.19. Split Windows Paths into Their Parts
- 8.20. Extract the Drive Letter from a Windows Path
- 8.21. Extract the Server and Share from a UNC Path
- 8.22. Extract the Folder from a Windows Path
- 8.23. Extract the Filename from a Windows Path
- 8.24. Extract the File Extension from a Windows Path
- 8.25. Strip Invalid Characters from Filenames
- 9. Markup and Data Formats
- Processing Markup and Data Formats with Regular Expressions
- 9.1. Find XML-Style Tags
- 9.2. Replace <b> Tags with <strong>
- 9.3. Remove All XML-Style Tags Except <em> and <strong>
- 9.4. Match XML Names
- 9.5. Convert Plain Text to HTML by Adding <p> and <br> Tags
- 9.6. Decode XML Entities
- 9.7. Find a Specific Attribute in XML-Style Tags
- 9.8. Add a cellspacing Attribute to <table> Tags That Do Not Already Include It
- 9.9. Remove XML-Style Comments
- 9.10. Find Words Within XML-Style Comments
- 9.11. Change the Delimiter Used in CSV Files
- 9.12. Extract CSV Fields from a Specific Column
- 9.13. Match INI Section Headers
- 9.14. Match INI Section Blocks
- 9.15. Match INI Name-Value Pairs
- Index
- About the Authors
- Colophon
- Copyright
As part of a cleanup routine for user input or other data, you want to replace repeated whitespace characters with a single space. Any tabs, line breaks, or other whitespace should also be replaced with a space.
To implement either of the following regular expressions, simply replace all matches with a single space character. Recipe 3.14 shows the code to do this.
A common text cleanup routine is to replace repeated whitespace characters with a single space. In HTML, for example, repeated whitespace is simply ignored when rendering a page (with a few exceptions). Removing repeated whitespace can therefore help to reduce the file size of some pages (or at least page sections) without any negative effects.
In this solution, any sequence of whitespace characters
(line breaks, tabs, spaces, etc.) is replaced with a single space.
Since the ‹+›
quantifier repeats the ‹s›
whitespace class one or more times, even a single tab character, for
example, will be replaced with a space. If you replaced the ‹+› with ‹{2,}›, only sequences of two or more whitespace
characters would be replaced. This could result in fewer replacements
and thus improved performance, but it could also leave behind tab
characters or line breaks that would otherwise be replaced with space
characters. The better approach, therefore, depends on what you’re
trying to accomplish.
This works exactly like the previous solution, except
that it leaves line breaks alone. Only spaces, tabs, and no-break
spaces are replaced. HTML no-break space entities ( ) are unaffected.
PCRE 7.2 and Perl 5.10 include the shorthand character class
‹h› that
you might prefer to use here since it is specifically designed to
match horizontal whitespace. It also matches some additional esoteric
horizontal whitespace characters.
Using ‹xA0› to
match no-break spaces in Ruby 1.9 may lead to an “invalid multibyte
escape” or other encoding related errors, since it references a
character beyond the ASCII range ‹x00› to ‹x7F›. Use ‹u00A0› instead.
Recipe 5.12 explains how to trim leading and trailing whitespace.
Techniques used in the regular expressions in this recipe are discussed in Chapter 2. Recipe 2.2 explains how to match nonprinting characters. Recipe 2.3 explains character classes. Recipe 2.12 explains repetition.
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