编程宝典：精通正则表达式解决问题

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from start to finish. We also thank Jeffrey Friedl, Zak Greant, Nikolaj Lindberg, and

Ian Morse for their careful technical reviews, which made this a more comprehensive

and accurate book.

xiv | Preface

History of the Term ‘Regular Expression’

The term regular expression comes from mathematics and computer science theory,

where it reflects a trait of mathematical expressions called regularity. Such an expres-

sion can be implemented in software using a deterministic finite automaton (DFA). A

DFA is a finite state machine that doesn’t use backtracking.

The text patterns used by the earliest grep tools were regular expressions in the math-

ematical sense. Though the name has stuck, modern-day Perl-style regular expressions

are not regular expressions at all in the mathematical sense. They’re implemented with

a nondeterministic finite automaton (NFA). You will learn all about backtracking

shortly. All a practical programmer needs to remember from this note is that some ivory

tower computer scientists get upset about their well-defined terminology being over-

loaded with technology that’s far more useful in the real world.

If you use regular expressions with skill, they simplify many programming and text

processing tasks,

and allow many that wouldn’t be at all feasible without the regular

expressions. You would need dozens if not hundreds of lines of procedural code to

extract all email addresses from a document—code that is tedious to write and hard to

maintain. But with the proper regular expression, as shown in Recipe 4.1, it takes just

a few lines of code, or maybe even one line.

But if you try to do too much with just one regular expression, or use regexes where

they’re not really appropriate, you’ll find out why some people say:

Some people, when confronted with a problem, think “I know, I’ll use regular expres-

sions.” Now they have two problems.

The second problem those people have is that they didn’t read the owner’s manual,

which you are holding now. Read on. Regular expressions are a powerful tool. If your

job involves manipulating or extracting text on a computer, a firm grasp of regular

expressions will save you plenty of overtime.

Many Flavors of Regular Expressions

All right, the title of the previous section was a lie. We didn’t define what regular

expressions are. We can’t. There is no official standard that defines exactly which text

patterns are regular expressions and which aren’t. As you can imagine, every designer

of programming languages and every developer of text processing applications has a

different idea of exactly what a regular expression should be. So now we’re stuck with

a whole palate of regular expression flavors.

Fortunately, most designers and developers are lazy. Why create something totally new

when you can copy what has already been done? As a result, all modern regular ex-

pression flavors, including those discussed in this book, can trace their history back to

Jeffrey Friedl traces the history of this quote in his blog at http://regex.info/blog/2006-09-15/247.

2 | Chapter 1: Introduction to Regular Expressions

the Perl programming language. We call these flavors Perl-style regular expressions.

Their regular expression syntax is very similar, and mostly compatible, but not com-

pletely so.

Writers are lazy, too. We’ll usually type regex or regexp to denote a single regular

expression, and regexes to denote the plural.

Regex flavors do not correspond one-to-one with programming languages. Scripting

languages tend to have their own, built-in regular expression flavor. Other program-

ming languages rely on libraries for regex support. Some libraries are available for mul-

tiple languages, while certain languages can draw on a choice of different libraries.

This introductory chapter deals with regular expression flavors only and completely

ignores any programming considerations. Chapter 3 begins the code listings, so you

can peek ahead to “Programming Languages and Regex Flavors” in Chapter 3 to find

out which flavors you’ll be working with. But ignore all the programming stuff for now.

The tools listed in the next section are an easier way to explore the regex syntax through

“learning by doing.”

Regex Flavors Covered by This Book

For this book, we selected the most popular regex flavors in use today. These are all

Perl-style regex flavors. Some flavors have more features than others. But if two flavors

have the same feature, they tend to use the same syntax. We’ll point out the few an-

noying inconsistencies as we encounter them.

All these regex flavors are part of programming languages and libraries that are in active

development. The list of flavors tells you which versions this book covers. Further along

in the book, we mention the flavor without any versions if the presented regex works

the same way with all flavors. This is almost always the case. Aside from bug fixes that

affect corner cases, regex flavors tend not to change, except to add features by giving

new meaning to syntax that was previously treated as an error:

Perl

Perl’s built-in support for regular expressions is the main reason why regexes are

popular today. This book covers Perl 5.6, 5.8, and 5.10.

Many applications and regex libraries that claim to use Perl or Perl-compatible

regular expressions in reality merely use Perl-style regular expressions. They use a

regex syntax similar to Perl’s, but don’t support the same set of regex features.

Quite likely, they’re using one of the regex flavors further down this list. Those

flavors are all Perl-style.

PCRE

PCRE is the “Perl-Compatible Regular Expressions” C library developed by Philip

Hazel. You can download this open source library at http://www.pcre.org. This

book covers versions 4 through 7 of PCRE.

Regular Expressions Defined | 3

Though PCRE claims to be Perl-compatible, and probably is more than any other

flavor in this book, it really is just Perl-style. Some features, such as Unicode sup-

port, are slightly different, and you can’t mix Perl code into your regex, as Perl itself

allows.

Because of its open source license and solid programming, PCRE has found its way

into many programming languages and applications. It is built into PHP and wrap-

ped into numerous Delphi components. If an application claims to support “Perl-

compatible” regular expressions without specifically listing the actual regex flavor

being used, it’s likely PCRE.

.NET

The Microsoft .NET Framework provides a full-featured Perl-style regex flavor

through the System.Text.RegularExpressions package. This book covers .NET

versions 1.0 through 3.5. Strictly speaking, there are only two versions of

System.Text.RegularExpressions: 1.0 and 2.0. No changes were made to the Regex

classes in .NET 1.1, 3.0, and 3.5.

Any .NET programming language, including C#, VB.NET, Delphi for .NET, and

even COBOL.NET, has full access to the .NET regex flavor. If an application de-

veloped with .NET offers you regex support, you can be quite certain it uses

the .NET flavor, even if it claims to use “Perl regular expressions.” A glaring ex-

ception is Visual Studio (VS) itself. The VS integrated development environment

(IDE) still uses the same old regex flavor it has had from the beginning, which is

not Perl-style at all.

Java

Java 4 is the first Java release to provide built-in regular expression support through

the java.util.regex package. It has quickly eclipsed the various third-party regex

libraries for Java. Besides being standard and built in, it offers a full-featured Perl-

style regex flavor and excellent performance, even when compared with applica-

tions written in C. This book covers the java.util.regex package in Java 4, 5, and

If you’re using software developed with Java during the past few years, any regular

expression support it offers likely uses the Java flavor.

JavaScript

In this book, we use the term JavaScript to indicate the regular expression flavor

defined in version 3 of the ECMA-262 standard. This standard defines the

ECMAScript programming language, which is better known through its JavaScript

and JScript implementations in various web browsers. Internet Explorer 5.5

through 8.0, Firefox, Opera, and Safari all implement Edition 3 of ECMA-262.

However, all browsers have various corner case bugs causing them to deviate from

the standard. We point out such issues in situations where they matter.

If a website allows you to search or filter using a regular expression without waiting

for a response from the web server, it uses the JavaScript regex flavor, which is the

4 | Chapter 1: Introduction to Regular Expressions

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