精通正则表达式第3版：文本处理利器

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"Mastering Regular Expressions, 3rd Edition, by Jeffrey E. F. Friedl, 是一本关于正则表达式的权威指南，由O'Reilly出版社于2006年8月出版，适用于各种编程语言和工具，如Perl、Python、Ruby、Java、VB.NET、C#、PHP以及MySQL等。本书旨在帮助读者深入理解和熟练掌握正则表达式这一强大的文本处理工具。无论你是初学者还是已经有一定经验的用户，这本书都将提供前所未有的详细讲解和广泛覆盖的知识点。即使你自认为对正则表达式了如指掌，本书也将为你带来新的洞察和启示。正则表达式是处理文本和数据的核心技术，它们能让你实现复杂且微妙的文本处理任务，这些任务在以前可能被认为无法自动化。通过正则表达式，你可以节省大量时间和精力，解决各种各样的问题。一旦你掌握了正则表达式，你将能够构建出优雅的解决方案。书中涵盖了正则表达式的各个方面，包括基础语法、元字符、量词、分组与捕获、选择、重复、位置匹配、后向引用、条件表达式、正向预查、反向预查、模式修饰符、回溯控制、性能优化，以及在不同环境下的使用技巧。此外，书中还包含了大量实例和练习，帮助读者加深理解并提升实践能力。本书的542页内容不仅提供了理论知识，还有实际应用案例，旨在帮助读者从理论到实践全面掌握正则表达式。通过学习本书，你将能够更好地利用正则表达式来解决日常的编程和数据处理挑战，提高工作效率，并为你的代码添加更多的灵活性和功能。"

1.2. Regular Expressions as a Language

Unless you've had some experience with regular expressions, you won't understand the regular

expression

^(From|Subject):

from the last example, but there's nothing magic about it. For

that matter, there is nothing magic about magic. The magician merely understands something

simple which doesn't appear to be simple or natural to the untrained audience. Once you learn

how to hold a card while making your hand look empty, you only need practice before you,

too, can "do magic." Like a foreign language once you learn it, it stops sounding like gibberish.

1.2.1. The Filename Analogy

Since you have decided to use this book, you probably have at least some idea of just what a

"regular expression" is. Even if you don't, you are almost certainly already familiar with the

basic concept.

You know that report.txt is a specific filename, but if you have had any experience with Unix

or DOS/Windows, you also know that the pattern "

*.txt

" can be used to select multiple files.

With filename patterns like this (called file globs or wildcards), a few characters have special

meaning. The star means "match anything," and a question mark means "match any one

character." So, with the file glob "

*.txt

," we start with a match-anything

and end with

the literal

.txt

, so we end up with a pattern that means "select the files whose names

start with anything and end with

.txt

Most systems provide a few additional special characters, but, in general, these filename

patterns are limited in expressive power. This is not much of a shortcoming because the scope

of the problem (to provide convenient ways to specify groups of files) is limited, well, simply

to filenames.

On the other hand, dealing with general text is a much larger problem. Prose and poetry,

program listings, reports, HTML, code tables, word lists... you name it, if a particular need is

specific enough, such as "selecting files," you can develop some kind of specialized scheme or

tool to help you accomplish it. However, over the years, a generalized pattern language has

developed, which is powerful and expressive for a wide variety of uses. Each program

implements and uses them differently, but in general, this powerful pattern language and

the patterns themselves are called regular expressions.

1.2.2. The Language Analogy

Full regular expressions are composed of two types of characters. The special characters (like

the * from the filename analogy) are called metacharacters, while the rest are called literal,

or normal text characters. What sets regular expressions apart from filename patterns are the

advanced expressive powers that their metacharacters provide. Filename patterns provide

limited metacharacters for limited needs, but a regular expression "language" provides rich and

expressive metacharacters for advanced uses.

It might help to consider regular expressions as their own language, with literal text acting as

the words and metacharacters as the grammar. The words are combined with grammar

according to a set of rules to create an expression that communicates an idea. In the email

example, the expression I used to find lines beginning with '

From:

' or '

Subject:

' was

. The metacharacters are underlined; we'll get to their interpretation

soon.

As with learning any other language, regular expressions might seem intimidating at first. This

is why it seems like magic to those with only a superficial understanding, and perhaps

completely unapproachable to those who have never seen it at all. But, just as

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[ ]

would soon become clear to a student of Japanese, the regular

expression in

[ ]

"Regular expressions are easy!" A somew hat humorous comment about this: as Chapter 3 explains, the term regular

expression originally comes from formal algebra. When people ask me w hat my book is about, the answ er "regular expressions"

draw s a blank face if they are not already familiar w ith the concept. The Japanese w ord for regular expression, , means

as little to the average Japanese as its English counterpart, but my reply in Japanese usually draw s a bit more than a blank stare.

You see, the "regular" part is unfortunately pronounced identically to a much more common w ord, a medical term for "reproductive

organs." You can only imagine w hat flashes through their minds until I explain!

s!<emphasis>([0-9]+(\.[0-9]+){3})</emphasis>!<inet>$1</inet>!

will soon become crystal clear to you, too.

This example is from a Perl language script that my editor used to modify a manuscript. The

author had mistakenly used the typesetting tag

to mark Internet IP addresses

(which are sets of periods and numbers that look like

209.204.146.22

). The incantation uses

Perl's text-substitution command with the regular expression

to replace such tags with the appropriate

<inet>

tag, while leaving other uses of

alone. In later chapters, you'll learn all the details of exactly how this type of incantation is

constructed, so you'll be able to apply the techniques to your own needs, with your own

application or programming language.

1.2.2.1. The goal of this book

The chance that you will ever want to replace

tags with

<inet>

tags is small, but it

is very likely that you will run into similar "replace this with that" problems. The goal of this

book is not to teach solutions to specific problems, but rather to teach you how to think

regular expressions so that you will be able to conquer whatever problem you may face.

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1.3. The Regular-Expression Frame of Mind

As we'll soon see, complete regular expressions are built up from small building-block units.

Each individual building block is quite simple, but since they can be combined in an infinite

number of ways, knowing how to combine them to achieve a particular goal takes some

experience. So, this chapter provides a quick overview of some regular-expression concepts.

It doesn't go into much depth, but provides a basis for the rest of this book to build on, and

sets the stage for important side issues that are best discussed before we delve too deeply

into the regular expressions themselves.

While some examples may seem silly (because some are silly), they represent the kind of tasks

that you will want to do you just might not realize it yet. If each point doesn't seem to make

sense, don't worry too much. Just let the gist of the lessons sink in. That's the goal of this

chapter.

1.3.1. If You Have Some Regular-Expression Experience

If you're already familiar with regular expressions, much of this overview will not be new, but

please be sure to at least glance over it anyway. Although you may be aware of the basic

meaning of certain metacharacters, perhaps some of the ways of thinking about and looking

at regular expressions will be new.

Just as there is a difference between playing a musical piece well and making music, there is

a difference between knowing about regular expressions and really understanding them. Some

of the lessons present the same information that you are already familiar with, but in ways

that may be new and which are the first steps to really understanding.

1.3.2. Searching Text Files: Egrep

Finding text is one of the simplest uses of regular expressionsmany text editors and word

processors allow you to search a document using a regular-expression pattern. Even simpler is

the utility egrep. Give egrep a regular expression and some files to search, and it attempts to

match the regular expression to each line of each file, displaying only those lines in which a

match is found. egrep is freely available for many systems, including DOS, MacOS, Windows,

Unix, and so on. See this book's web site, http://regex.info, for links on how to obtain a copy

of egrep for your system.

Returning to the email example from page 3, the command I actually used to generate a

makeshift table of contents from the email file is shown in Figure 1-1. egrep interprets the

first command-line argument as a regular expression, and any remaining arguments as the

file(s) to search. Note, however, that the single quotes shown in Figure 1-1 are not part of

the regular expression, but are needed by my command shell.

[ ]

When using egrep, I usually

wrap the regular expression with single quotes. Exactly which characters are special, in what

contexts, to whom (to the regular-expression, or to the tool), and in what order they are

interpreted are all issues that grow in importance when you move to regular-expression use in

fullfledged programming languagessomething we'll see starting in the next chapter.

[ ]

The command shell is the part of the system that accepts your typed commands and actually executes the programs you

request. With the shell I use, the single quotes serve to group the command argument, telling the shell not to pay too much attention

to w hat's inside. If I didn't use them, the shell might think, for example, a '

' that I intended to be part of the regular expression w as

really part of a filename pattern that it should interpret. I don't w ant that to happen, so I use the quotes to "hide" the metacharacters

from the shell. Window s users of COMMAND.COM or CMD.EXE should probably use double quotes instead.

Figure 1-1. Invoking egrep from the command line

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1.4. Egrep Metacharacters

Let's start to explore some of the egrep metacharacters that supply its regular-expression

power. I'll go over them quickly with a few examples, leaving the detailed examples and

descriptions for later chapters.

Typographical Conventions Before we begin, please make sure to review the typographical

conventions explained in the preface, on page xxi. This book forges a bit of new ground in the

area of typesetting, so some of my notations may be unfamiliar at first.

1.4.1. Start and End of the Line

Probably the easiest metacharacters to understand are

(caret) and

(dollar), which

represent the start and end, respectively, of the line of text as it is being checked. As we've

seen, the regular expression

cat

finds

anywhere on the line, but

^cat

matches only

if the

is at the beginning of the linethe

is used to effectively anchor the match (of

the rest of the regular expression) to the start of the line. Similarly,

cat$

finds

only at

the end of the line, such as a line ending with

scat

It's best to get into the habit of interpreting regular expressions in a rather literal way. For

example, don't think

^cat

matches a line with

cat

at the beginning

but rather:

^cat

matches if you have the beginning of a line, followed immediately by

, followed

immediately by

, followed immediately by

They both end up meaning the same thing, but reading it the more literal way allows you to

intrinsically understand a new expression when you see it. How would egrep interpret

^cat$

, or even simply

alone?

❖

Turn the page to check your interpretations.

The caret and dollar are special in that they match a position in the line rather than any

actual text characters themselves. Of course, there are various ways to actually match real

text. Besides providing literal characters like

cat

in your regular expression, you can also

use some of the items discussed in the next few sections.

1.4.2. Character Classes

1.4.2.1. Matching any one of several characters

Let's say you want to search for "grey," but also want to find it if it were spelled "gray." The

regular-expression construct

[

⋯

]

, usually called a character class, lets you list the

characters you want to allow at that point in the match. While

matches just an

, and

matches just an

, the regular expression

[ea]

matches either. So, then, consider

gr[ea]y

: this means to find "

, followed by

, followed by either an

or an

, all followed by

." Because I'm a really poor speller, I'm always using regular expressions like this against a

huge list of English words to figure out proper spellings. One I use often is

sep[ea]r[ea]te

because I can never remember whether the word is spelled "seperate," "separate," "separete,"

or what. The one that pops up in the list is the proper spelling; regular expressions to the

rescue.

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