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This book is about a powerful tool called “regular expressions”. It teaches you how

to use regular expressions to solve problems and get the most out of tools and

languages that provide them. Most documentation that mentions regular expres￾sions doesn’t even begin to hint at their power, but this book is about mastering

regular expressions.

Regular expressions are available in many types of tools (editors, word processors,

system tools, database engines, and such), but their power is most fully exposed

when available as part of a programming language. Examples include Java and

JScript, Visual Basic and VBScript, JavaScript and ECMAScript, C, C++, C#, elisp, Perl,

Python, Tcl, Ruby, PHP, sed, and awk. In fact, regular expressions are the very

heart of many programs written in some of these languages.

Ther e’s a good reason that regular expressions are found in so many diverse lan￾guages and applications: they are extr emely power ful. At a low level, a regular

expr ession describes a chunk of text. You might use it to verify a user’s input, or

perhaps to sift through large amounts of data. On a higher level, regular expres￾sions allow you to master your data. Control it. Put it to work for you. To master

regular expressions is to master your data.

The Need for This Book

I finished the first edition of this book in late 1996, and wrote it simply because

ther e was a need. Good documentation on regular expressions just wasn’t avail￾able, so most of their power went untapped. Regular-expr ession documentation

was available, but it centered on the “low-level view.” It seemed to me that they

wer e analogous to showing someone the alphabet and expecting them to learn to

speak.

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2 July 2006 02:01

xviii Preface

The five and a half years between the first and second editions of this book saw

the popular rise of the Internet, and, perhaps more than just coincidentally, a considerable expansion in the world of regular expressions. The regular expressions

of almost every tool and language became more power ful and expressive. Perl,

Python, Tcl, Java, and Visual Basic all got new regular-expr ession backends. New

languages with regular expression support, like PHP, Ruby, and C#, were developed and became popular. During all this time, the basic core of the book — how

to truly understand regular expressions and how to get the most from them —

remained as important and relevant as ever.

Yet, the first edition gradually started to show its age. It needed updating to reflect

the new languages and features, as well as the expanding role that regular expressions played in the Internet world. It was published in 2002, a year that saw the

landmark releases of java.util.regex, Micr osoft’s .NET Framework, and Perl 5.8.

They were all covered fully in the second edition. My one regr et with the second

edition was that it didn’t give more attention to PHP. In the four years since the

second edition was published, PHP has only grown in importance, so it became

imperative to correct that deficiency.

This third edition features enhanced PHP coverage in the early chapters, plus an all

new, expansive chapter devoted entirely to PHP regular expressions and how to

wield them effectively. Also new in this edition, the Java chapter has been rewritten and expanded considerably to reflect new features of Java 1.5 and Java 1.6.

Intended Audience

This book will interest anyone who has an opportunity to use regular expressions.

If you don’t yet understand the power that regular expressions can provide, you

should benefit greatly as a whole new world is opened up to you. This book

should expand your understanding, even if you consider yourself an accomplished

regular-expr ession expert. After the first edition, it wasn’t uncommon for me to

receive an email that started “I thought I knew regular expressions until I read

Mastering Regular Expressions. Now I do.”

Pr ogrammers working on text-related tasks, such as web programming, will find

an absolute gold mine of detail, hints, tips, and understanding that can be put to

immediate use. The detail and thoroughness is simply not found anywhere else.

Regular expressions are an idea— one that is implemented in various ways by various utilities (many, many more than are specifically presented in this book). If you

master the general concept of regular expressions, it’s a short step to mastering a

particular implementation. This book concentrates on that idea, so most of the

knowledge presented here transcends the utilities and languages used to present

the examples.