精通正则表达式：实战指南

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"Mastering Regular Expressions 是一本深入学习正则表达式的权威书籍，由 Jeffrey E.F. Friedl 撰写。这本书的第三版详细介绍了如何熟练掌握和运用正则表达式，对于面试和实际工作中的问题解决具有重要参考价值。" 正则表达式是一种强大的文本处理工具，广泛应用于数据验证、搜索、替换、提取等多种场景。在编程语言中，如JavaScript、Python、Java等，正则表达式是不可或缺的部分，能够帮助开发者高效地处理字符串。本书"Mastering Regular Expressions"分为三个主要部分： 1. 基础与概念：首先介绍了正则表达式的基本语法和元字符，包括`.`（匹配任意字符）、`^`（行首）、`$`（行尾）、`\d`（数字）、`\w`（字母数字字符）以及各种字符类。此外，还讨论了量词（如`*`、`+`、`?`和`{n,m}`）和分组机制。 2. 高级特性：进一步探讨更复杂的构造，如后向引用、非捕获组、预查、条件表达式和递归。这些特性使得正则表达式可以处理更为复杂的模式匹配需求。 3. 实现与性能：针对不同平台和语言的正则表达式引擎进行了对比，讲解了如何选择合适的引擎，以及如何优化正则表达式以提高性能。这一部分对于开发者在实际项目中应用正则表达式至关重要。书中通过丰富的实例来解释各个概念，帮助读者理解和掌握。同时，作者Jeffrey E.F. Friedl在书中还强调了正则表达式调试技巧，这对于避免错误和理解复杂表达式非常有帮助。此外，该书还涵盖了一些特殊场景的应用，比如URL和电子邮件地址的验证、日期和时间的匹配，以及XML和HTML文档的处理。对于面试而言，理解并能灵活运用这些内容可以显著提升面试者的技能水平。 "Mastering Regular Expressions"是一本深入而全面的正则表达式指南，无论你是初学者还是经验丰富的开发者，都能从中获得宝贵的知识和经验。通过学习这本书，你将能够更加熟练地玩转正则表达式，提高你的编程技能。

xx Preface

The introduction elevates the absolute novice to “issue-aware” novice. Readers

with a fair amount of experience can feel free to skim the early chapters, but I par-

ticularly recommend Chapter 3 even for the grizzled expert.

•Chapter 1, Intr oduction to Regular Expressions,isgear ed toward the complete

novice. I introduce the concept of regular expressions using the widely avail-

able program egr ep,and offer my perspective on how to think regular expres-

sions, instilling a solid foundation for the advanced concepts presented in later

chapters. Even readers with former experience would do well to skim this ﬁrst

chapter.

•Chapter 2, Extended Introductory Examples,looks at real text processing in a

pr ogramming language that has regular-expr ession support. The additional

examples provide a basis for the detailed discussions of later chapters, and

show additional important thought processes behind crafting advanced regular

expr essions. To provide a feel for how to “speak in regular expressions,”this

chapter takes a problem requiring an advanced solution and shows ways to

solve it using two unrelated regular-expr ession–wielding tools.

•Chapter 3, Overview of Regular Expression Features and Flavors,provides an

overview of the wide range of regular expressions commonly found in tools

today. Due to their turbulent history, current commonly-used regular-expr es-

sion ﬂavors can differ greatly. This chapter also takes a look at a bit of the his-

tory and evolution of regular expressions and the programs that use them. The

end of this chapter also contains the “Guide to the Advanced Chapters.”This

guide is your road map to getting the most out of the advanced material that

follows.

TheDetails

Once you have the basics down, it’s time to investigate the how and the why.Like

the “teach a man to ﬁsh” parable, truly understanding the issues will allow you to

apply that knowledge whenever and wherever regular expressions arefound.

•Chapter 4, The Mechanics of Expression Processing,ratchets up the pace sev-

eral notches and begins the central coreofthis book. It looks at the important

inner workings of how regular expression engines really work from a practi-

cal point of view. Understanding the details of how regular expressions are

handled goes a very long way toward allowing you to master them.

•Chapter 5, Practical Regex Techniques,then puts that knowledge to high-level,

practical use. Common (but complex) problems areexplor ed in detail, all with

the aim of expanding and deepening your regular-expr ession experience.

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•Chapter 6, Crafting an Efﬁcient Expression,looks at the real-life efﬁciency

ramiﬁcations of the regular expressions available to most programming lan-

guages. This chapter puts information detailed in Chapters 4 and 5 to use for

exploiting an engine’s strengths and stepping around its weaknesses.

Tool-Specific Infor mation

Once the lessons of Chapters 4, 5, and 6 areunder your belt, thereisusually little

to say about speciﬁc implementations. However,I’ve devoted an entirechapter to

each of four popular systems:

•Chapter 7, Perl,closely examines regular expressions in Perl, arguably the

most popular regular-expr ession–laden pr ogramming language in use today. It

has only four operators related to regular expressions, but their myriad of

options and special situations provides an extremely rich set of programming

options

—

and pitfalls. The very richness that allows the programmer to move

quickly from concept to program can be a mineﬁeld for the uninitiated. This

detailed chapter clears a path.

•Chapter 8, Java,looks in detail at the java.util.regex regular-expr ession

package, a standard part of the language since Java 1.4. The chapter’s primary

focus is on Java 1.5, but differ ences in both Java 1.4.2 and Java 1.6 arenoted.

•Chapter 9, .NET,isthe documentation for the .NET regular-expr ession library

that Microsoft neglected to provide. Whether using VB.NET,C#, C

,JScript,

VBscript, ECMAScript, or any of the other languages that use .NET components,

this chapter provides the details you need to employ .NET regular-expr essions

to the fullest.

•Chapter 10, PHP,provides a short introduction to the multiple regex engines

embedded within PHP,followed by a detailed look at the regex ﬂavor and API

of its preg regex suite, powered under the hood by the PCRE regex library.

Typog raphical Conventions

When doing (or talking about) detailed and complex text processing, being pre-

cise is important. The mereaddition or subtraction of a space can make a world of

dif ference, so I’ve used the following special conventions in typesetting this book:

• Aregular expression generally appears like

!this".Notice the thin corners

which ﬂag “this is a regular expression.”Literal text (such as that being

searched) generally appears like ‘this’. At times, I’ll leave offthe thin corners

or quotes when obviously unambiguous. Also, code snippets and screen shots

ar e always presented in their natural state, so the quotes and corners arenot

used in such cases.

Preface xxi

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xxii Preface

• Iuse visually distinct ellipses within literal text and regular expressions. For

example [˙˙˙] repr esents aset of squarebrackets with unspeciﬁed contents,

while [...] would be a set containing three periods.

• Without special presentation, it is virtually impossible to know how many

spaces arebetween the letters in “a b”,sowhen spaces appear in regular

expr essions and selected literal text, they arepresented with the ‘

’symbol.

This way, it will be clear that thereare exactly four spaces in ‘

b’.

• Ialso use visual tab, newline, and carriage-retur n characters:

aspace character

2 atab character

1 anewline character

| acarriage-r eturncharacter

• At times, I use underlining or shade the background to highlight parts of literal

text or a regular expression. In this example the underline shows whereinthe

text the expression actually matches:

Because

!cat" matches ‘It

indicat

your

cat

is˙˙˙’instead of the

word ‘

cat’, we realize . . .

In this example the underlines highlight what has just been added to an

expr ession under discussion:

To make this useful, we can wrap

!Subject;Date" with parentheses,

and append a colon and a space. This yields

Subject;Date):

• This book is full of details and examples, so I’ve included over 1,200 cross ref-

er ences to help you get the most out of it. They often appear in the text in a

“☞123” notation, which means “see page 123.”For example, it might appear

like “ ...isdescribed in Table 8-2 (

☞

367).”

Exer cises

Occasionally, and particularly in the early chapters, I’ll pose a question to highlight

the importance of the concept under discussion. They’renot therejust to take up

space; I really do want you to try them beforecontinuing. Please. So as not to

dilute their importance, I’ve sprinkled only a few throughout the entirebook. They

also serve as checkpoints: if they take morethan a few moments, it’s probably

best to go over the relevant section again beforecontinuing on.

To help entice you to actually think about these questions as you read them, I’ve

made checking the answers a breeze: just turnthe page. Answers to questions

marked with

❖

ar e always found by turning just one page. This way, they’reout

of sight while you think about the answer,but arewithin easy reach.

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xxiv Preface

Personal Comments and

Acknowledgments

Writing the ﬁrst edition of this book was a grueling task that took two and a half

years and the help of many people. After the toll it took on my health and sanity, I

pr omised that I’d never put myself through such an experience again.

Ihave many people to thank in helping me break that promise. Foremost is my

wife, Fumie. If you ﬁnd this book useful, thank her; without her support and

understanding, I’d have neither the strength nor sanity to undertake a task as ardu-

ous as the research, writing, and production of a book like this.

While researching and writing this book, many people helped educate me on lan-

guages or systems I didn’t know, and morestill reviewed and corrected drafts as

the manuscripts developed.

In particular,I’d like to thank my brother,Stephen Friedl, for his meticulous and

detailed reviews along the way. (Besides being an excellent technical reviewer,

he’s also an accomplished writer,known for his well-researched “Tech Tips,”avail-

able at http://www.unixwiz.net/)

I’d also like to thank Zak Greant, Ian Morse, Philip Hazel, Stuart Gill, William F.

Maton, and my editor,Andy Oram.

Special thanks for providing an insider’s look at Java go to Mike “madbot”

McCloskey (formerly at Sun Microsystems, now at Google), and Mark Reinhold

and Dr.Clif f Click, both of Sun Microsystems. For .NET insight, I’d like to thank

Micr osoft’s David Gutierrez, Kit George, and Ryan Byington. I thank Andrei

Zmievski of Yahoo! for providing insights into PHP.

I’d like to thank Dr.Ken Lunde of Adobe Systems, who created custom characters

and fonts for a number of the typographical aspects of this book. The Japanese

characters arefromAdobe Systems’ Heisei Mincho W3 typeface, while the Korean

is from the Korean Ministry of Cultureand Sports Munhwa typeface. It’s also Ken

who originally gave me the guiding principle that governs my writing: “you do the

research so your readers don’t have to.”

For help in setting up the server for http://r egex.info,I’d like to thank Jeffr ey

Papen and Peak Web Hosting (http://www.PeakWebhosting.com/).

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