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SECOND EDITION

Regular Expressions Cookbook

Jan Goyvaerts and Steven Levithan

Beijing Cambridge Farnham Köln Sebastopol Tokyo

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Regular Expressions Cookbook, Second Edition

by Jan Goyvaerts and Steven Levithan

Copyright © 2012 Jan Goyvaerts, Steven Levithan. All rights reserved.

Printed in the United States of America.

Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472.

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Editor: Andy Oram

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Cover Designer: Karen Montgomery

Interior Designer: David Futato

Illustrator: Rebecca Demarest

August 2012: Second Edition.

Revision History for the Second Edition:

2012-08-10 First release

See http://oreilly.com/catalog/errata.csp?isbn=9781449319434 for release details.

Nutshell Handbook, the Nutshell Handbook logo, and the O’Reilly logo are registered trademarks of

O’Reilly Media, Inc. Regular Expressions Cookbook, the image of a musk shrew, and related trade dress

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While every precaution has been taken in the preparation of this book, the publisher and authors assume

no responsibility for errors or omissions, or for damages resulting from the use of the information con￾tained herein.

ISBN: 978-1-449-31943-4

[LSI]

1344629030

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Table of Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

1. Introduction to Regular Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Regular Expressions Defined 1

Search and Replace with Regular Expressions 6

Tools for Working with Regular Expressions 8

2. Basic Regular Expression Skills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.1 Match Literal Text 28

2.2 Match Nonprintable Characters 30

2.3 Match One of Many Characters 33

2.4 Match Any Character 38

2.5 Match Something at the Start and/or the End of a Line 40

2.6 Match Whole Words 45

2.7 Unicode Code Points, Categories, Blocks, and Scripts 48

2.8 Match One of Several Alternatives 62

2.9 Group and Capture Parts of the Match 63

2.10 Match Previously Matched Text Again 66

2.11 Capture and Name Parts of the Match 68

2.12 Repeat Part of the Regex a Certain Number of Times 72

2.13 Choose Minimal or Maximal Repetition 75

2.14 Eliminate Needless Backtracking 78

2.15 Prevent Runaway Repetition 81

2.16 Test for a Match Without Adding It to the Overall Match 84

2.17 Match One of Two Alternatives Based on a Condition 91

2.18 Add Comments to a Regular Expression 93

2.19 Insert Literal Text into the Replacement Text 95

2.20 Insert the Regex Match into the Replacement Text 98

2.21 Insert Part of the Regex Match into the Replacement Text 99

2.22 Insert Match Context into the Replacement Text 103

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3. Programming with Regular Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Programming Languages and Regex Flavors 105

3.1 Literal Regular Expressions in Source Code 111

3.2 Import the Regular Expression Library 117

3.3 Create Regular Expression Objects 119

3.4 Set Regular Expression Options 126

3.5 Test If a Match Can Be Found Within a Subject String 133

3.6 Test Whether a Regex Matches the Subject String Entirely 140

3.7 Retrieve the Matched Text 144

3.8 Determine the Position and Length of the Match 151

3.9 Retrieve Part of the Matched Text 156

3.10 Retrieve a List of All Matches 164

3.11 Iterate over All Matches 169

3.12 Validate Matches in Procedural Code 176

3.13 Find a Match Within Another Match 179

3.14 Replace All Matches 184

3.15 Replace Matches Reusing Parts of the Match 192

3.16 Replace Matches with Replacements Generated in Code 197

3.17 Replace All Matches Within the Matches of Another Regex 203

3.18 Replace All Matches Between the Matches of Another Regex 206

3.19 Split a String 211

3.20 Split a String, Keeping the Regex Matches 219

3.21 Search Line by Line 224

3.22 Construct a Parser 228

4. Validation and Formatting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

4.1 Validate Email Addresses 243

4.2 Validate and Format North American Phone Numbers 249

4.3 Validate International Phone Numbers 254

4.4 Validate Traditional Date Formats 256

4.5 Validate Traditional Date Formats, Excluding Invalid Dates 260

4.6 Validate Traditional Time Formats 266

4.7 Validate ISO 8601 Dates and Times 269

4.8 Limit Input to Alphanumeric Characters 275

4.9 Limit the Length of Text 278

4.10 Limit the Number of Lines in Text 283

4.11 Validate Affirmative Responses 288

4.12 Validate Social Security Numbers 289

4.13 Validate ISBNs 292

4.14 Validate ZIP Codes 300

4.15 Validate Canadian Postal Codes 301

4.16 Validate U.K. Postcodes 302

4.17 Find Addresses with Post Office Boxes 303

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4.18 Reformat Names From “FirstName LastName” to “LastName,

FirstName” 305

4.19 Validate Password Complexity 308

4.20 Validate Credit Card Numbers 317

4.21 European VAT Numbers 323

5. Words, Lines, and Special Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331

5.1 Find a Specific Word 331

5.2 Find Any of Multiple Words 334

5.3 Find Similar Words 336

5.4 Find All Except a Specific Word 340

5.5 Find Any Word Not Followed by a Specific Word 342

5.6 Find Any Word Not Preceded by a Specific Word 344

5.7 Find Words Near Each Other 348

5.8 Find Repeated Words 355

5.9 Remove Duplicate Lines 358

5.10 Match Complete Lines That Contain a Word 362

5.11 Match Complete Lines That Do Not Contain a Word 364

5.12 Trim Leading and Trailing Whitespace 365

5.13 Replace Repeated Whitespace with a Single Space 369

5.14 Escape Regular Expression Metacharacters 371

6. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375

6.1 Integer Numbers 375

6.2 Hexadecimal Numbers 379

6.3 Binary Numbers 381

6.4 Octal Numbers 383

6.5 Decimal Numbers 384

6.6 Strip Leading Zeros 385

6.7 Numbers Within a Certain Range 386

6.8 Hexadecimal Numbers Within a Certain Range 392

6.9 Integer Numbers with Separators 395

6.10 Floating-Point Numbers 396

6.11 Numbers with Thousand Separators 399

6.12 Add Thousand Separators to Numbers 401

6.13 Roman Numerals 406

7. Source Code and Log Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409

7.1 Keywords 409

7.2 Identifiers 412

7.3 Numeric Constants 413

7.4 Operators 414

7.5 Single-Line Comments 415

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7.6 Multiline Comments 416

7.7 All Comments 417

7.8 Strings 418

7.9 Strings with Escapes 421

7.10 Regex Literals 423

7.11 Here Documents 425

7.12 Common Log Format 426

7.13 Combined Log Format 430

7.14 Broken Links Reported in Web Logs 431

8. URLs, Paths, and Internet Addresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435

8.1 Validating URLs 435

8.2 Finding URLs Within Full Text 438

8.3 Finding Quoted URLs in Full Text 440

8.4 Finding URLs with Parentheses in Full Text 442

8.5 Turn URLs into Links 444

8.6 Validating URNs 445

8.7 Validating Generic URLs 447

8.8 Extracting the Scheme from a URL 453

8.9 Extracting the User from a URL 455

8.10 Extracting the Host from a URL 457

8.11 Extracting the Port from a URL 459

8.12 Extracting the Path from a URL 461

8.13 Extracting the Query from a URL 464

8.14 Extracting the Fragment from a URL 465

8.15 Validating Domain Names 466

8.16 Matching IPv4 Addresses 469

8.17 Matching IPv6 Addresses 472

8.18 Validate Windows Paths 486

8.19 Split Windows Paths into Their Parts 489

8.20 Extract the Drive Letter from a Windows Path 494

8.21 Extract the Server and Share from a UNC Path 495

8.22 Extract the Folder from a Windows Path 496

8.23 Extract the Filename from a Windows Path 498

8.24 Extract the File Extension from a Windows Path 499

8.25 Strip Invalid Characters from Filenames 500

9. Markup and Data Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503

Processing Markup and Data Formats with Regular Expressions 503

9.1 Find XML-Style Tags 510

9.2 Replace <b> Tags with <strong> 526

9.3 Remove All XML-Style Tags Except <em> and <strong> 530

9.4 Match XML Names 533

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9.5 Convert Plain Text to HTML by Adding <p> and <br> Tags 539

9.6 Decode XML Entities 543

9.7 Find a Specific Attribute in XML-Style Tags 545

9.8 Add a cellspacing Attribute to <table> Tags That Do Not Already

Include It 550

9.9 Remove XML-Style Comments 553

9.10 Find Words Within XML-Style Comments 558

9.11 Change the Delimiter Used in CSV Files 562

9.12 Extract CSV Fields from a Specific Column 565

9.13 Match INI Section Headers 569

9.14 Match INI Section Blocks 571

9.15 Match INI Name-Value Pairs 572

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575

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Preface

Over the past decade, regular expressions have experienced a remarkable rise in pop￾ularity. Today, all the popular programming languages include a powerful regular ex￾pression library, or even have regular expression support built right into the language.

Many developers have taken advantage of these regular expression features to provide

the users of their applications the ability to search or filter through their data using a

regular expression. Regular expressions are everywhere.

Many books have been published to ride the wave of regular expression adoption. Most

do a good job of explaining the regular expression syntax along with some examples

and a reference. But there aren’t any books that present solutions based on regular

expressions to a wide range of real-world practical problems dealing with text on a

computer and in a range of Internet applications. We, Steve and Jan, decided to fill that

need with this book.

We particularly wanted to show how you can use regular expressions in situations

where people with limited regular expression experience would say it can’t be done, or

where software purists would say a regular expression isn’t the right tool for the job.

Because regular expressions are everywhere these days, they are often a readily available

tool that can be used by end users, without the need to involve a team of programmers.

Even programmers can often save time by using a few regular expressions for informa￾tion retrieval and alteration tasks that would take hours or days to code in procedural

code, or that would otherwise require a third-party library that needs prior review and

management approval.

Caught in the Snarls of Different Versions

As with anything that becomes popular in the IT industry, regular expressions come

in many different implementations, with varying degrees of compatibility. This has

resulted in many different regular expression flavors that don’t always act the same

way, or work at all, on a particular regular expression.

Many books do mention that there are different flavors and point out some of the

differences. But they often leave out certain flavors here and there—particularly

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when a flavor lacks certain features—instead of providing alternative solutions or

workarounds. This is frustrating when you have to work with different regular expres￾sion flavors in different applications or programming languages.

Casual statements in the literature, such as “everybody uses Perl-style regular expres￾sions now,” unfortunately trivialize a wide range of incompatibilities. Even “Perl-style”

packages have important differences, and meanwhile Perl continues to evolve. Over￾simplified impressions can lead programmers to spend half an hour or so fruitlessly

running the debugger instead of checking the details of their regular expression imple￾mentation. Even when they discover that some feature they were depending on is not

present, they don’t always know how to work around it.

This book is the first book on the market that discusses the most popular and feature￾rich regular expression flavors side by side, and does so consistently throughout the

book.

Intended Audience

You should read this book if you regularly work with text on a computer, whether that’s

searching through a pile of documents, manipulating text in a text editor, or developing

software that needs to search through or manipulate text. Regular expressions are an

excellent tool for the job. Regular Expressions Cookbook teaches you everything you

need to know about regular expressions. You don’t need any prior experience what￾soever, because we explain even the most basic aspects of regular expressions.

If you do have experience with regular expressions, you’ll find a wealth of detail that

other books and online articles often gloss over. If you’ve ever been stumped by a regex

that works in one application but not another, you’ll find this book’s detailed and equal

coverage of seven of the world’s most popular regular expression flavors very valuable.

We organized the whole book as a cookbook, so you can jump right to the topics you

want to read up on. If you read the book cover to cover, you’ll become a world-class

chef of regular expressions.

This book teaches you everything you need to know about regular expressions and then

some, regardless of whether you are a programmer. If you want to use regular expres￾sions with a text editor, search tool, or any application with an input box labeled

“regex,” you can read this book with no programming experience at all. Most of the

recipes in this book have solutions purely based on one or more regular expressions.

If you are a programmer, Chapter 3 provides all the information you need to implement

regular expressions in your source code. This chapter assumes you’re familiar with the

basic language features of the programming language of your choice, but it does not

assume you have ever used a regular expression in your source code.

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Technology Covered

.NET, Java, JavaScript, PCRE, Perl, Python, and Ruby aren’t just back-cover buzz￾words. These are the seven regular expression flavors covered by this book. We cover

all seven flavors equally. We’ve particularly taken care to point out all the inconsisten￾cies that we could find between those regular expression flavors.

The programming chapter (Chapter 3) has code listings in C#, Java, JavaScript, PHP,

Perl, Python, Ruby, and VB.NET. Again, every recipe has solutions and explanations

for all eight languages. While this makes the chapter somewhat repetitive, you can easily

skip discussions on languages you aren’t interested in without missing anything you

should know about your language of choice.

Organization of This Book

The first three chapters of this book cover useful tools and basic information that give

you a basis for using regular expressions; each of the subsequent chapters presents a

variety of regular expressions while investigating one area of text processing in depth.

Chapter 1, Introduction to Regular Expressions, explains the role of regular expressions

and introduces a number of tools that will make it easier to learn, create, and debug

them.

Chapter 2, Basic Regular Expression Skills, covers each element and feature of regular

expressions, along with important guidelines for effective use. It forms a complete tu￾torial to regular expressions.

Chapter 3, Programming with Regular Expressions, specifies coding techniques and

includes code listings for using regular expressions in each of the programming lan￾guages covered by this book.

Chapter 4, Validation and Formatting, contains recipes for handling typical user input,

such as dates, phone numbers, and postal codes in various countries.

Chapter 5, Words, Lines, and Special Characters, explores common text processing

tasks, such as checking for lines that contain or fail to contain certain words.

Chapter 6, Numbers, shows how to detect integers, floating-point numbers, and several

other formats for this kind of input.

Chapter 7, Source Code and Log Files, provides building blocks for parsing source code

and other text file formats, and shows how you can process log files with regular

expressions.

Chapter 8, URLs, Paths, and Internet Addresses, shows you how to take apart and

manipulate the strings commonly used on the Internet and Windows systems to find

things.

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Chapter 9, Markup and Data Formats, covers the manipulation of HTML, XML,

comma-separated values (CSV), and INI-style configuration files.

Conventions Used in This Book

The following typographical conventions are used in this book:

Italic

Indicates new terms, URLs, email addresses, filenames, and file extensions.

Constant width

Used for program listings, program elements such as variable or function names,

values returned as the result of a regular expression replacement, and subject or

input text that is applied to a regular expression. This could be the contents of a

text box in an application, a file on disk, or the contents of a string variable.

Constant width italic

Shows text that should be replaced with user-supplied values or by values deter￾mined by context.

‹Regular●expression›

Represents a regular expression, standing alone or as you would type it into the

search box of an application. Spaces in regular expressions are indicated with gray

circles to make them more obvious. Spaces are not indicated with gray circles in

free-spacing mode because this mode ignores spaces.

«Replacement●text»

Represents the text that regular expression matches will be replaced within a

search-and-replace operation. Spaces in replacement text are indicated with gray

circles to make them more obvious.

Matched text

Represents the part of the subject text that matches a regular expression.

⋯

A gray ellipsis in a regular expression indicates that you have to “fill in the blank”

before you can use the regular expression. The accompanying text explains what

you can fill in.

CR , LF , and CRLF

CR, LF, and CRLF in boxes represent actual line break characters in strings, rather

than character escapes such as \r, \n, and \r\n. Such strings can be created by

pressing Enter in a multiline edit control in an application, or by using multiline

string constants in source code such as verbatim strings in C# or triple-quoted

strings in Python.

↵

The return arrow, as you may see on the Return or Enter key on your keyboard,

indicates that we had to break up a line to make it fit the width of the printed page.

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When typing the text into your source code, you should not press Enter, but instead

type everything on a single line.

This icon signifies a tip, suggestion, or general note.

This icon indicates a warning or caution.

Using Code Examples

This book is here to help you get your job done. In general, you may use the code in

this book in your programs and documentation. You do not need to contact us for

permission unless you’re reproducing a significant portion of the code. For example,

writing a program that uses several chunks of code from this book does not require

permission. Selling or distributing a CD-ROM of examples from O’Reilly books does

require permission. Answering a question by citing this book and quoting example

code does not require permission. Incorporating a significant amount of example code

from this book into your product’s documentation does require permission.

We appreciate, but do not require, attribution. An attribution usually includes the title,

author, publisher, and ISBN. For example: “Regular Expressions Cookbook by Jan

Goyvaerts and Steven Levithan. Copyright 2012 Jan Goyvaerts and Steven Levithan,

978-1-449-31943-4.”

If you feel your use of code examples falls outside fair use or the permission given here,

feel free to contact us at [email protected].

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