精通正则表达式：文本处理利器

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"AW.Learning.Regular.Expressions 是一本旨在教授初学者如何掌握正则表达式的书籍。作者 Ben Forta 通过简单易懂的方式，逐步引导读者从基础的文本匹配到更复杂的正则表达式技巧，如回溯引用、条件评估和前瞻处理。这本书不仅教你如何在各种编程语言和平台中使用正则表达式，还涵盖了在编辑器中的搜索替换操作、网页应用中的文本处理，以及使用命令行工具进行文件搜索。书中还涉及了针对不同数据类型（如电话号码、邮政编码、社保号、IP地址、URL、电子邮件地址和信用卡号）的正则表达式应用。" 正则表达式是一种强大的文本处理工具，它允许我们用简洁的模式来匹配、查找、替换和提取文本中的特定模式。在本书中，你将学习到： 1. **基础概念**：了解正则表达式的本质，如何读取和理解这些模式，以及它们在解决问题时的重要性。 2. **基本元素**：学习如何使用文本字面量和元字符构建强大的搜索模式。元字符是具有特殊含义的字符，例如`.`用于匹配任何字符，`*`表示前一个字符可以出现零次或多次。 3. **高级特性**：探索回溯引用，它允许你在匹配模式中引用先前捕获的分组；学习前瞻（lookahead）和后瞻（lookbehind），这些技术可以帮助你在匹配文本时检查其前后是否有特定的模式。 4. **搜索与替换**：掌握在专业编辑工具（如Sublime Text、Visual Studio Code等）中执行高效搜索和替换操作的方法。 5. **Web应用集成**：了解如何将正则表达式应用于网页表单验证和文本处理，提升用户体验并确保数据有效性。 6. **命令行工具**：学习如何结合grep、egrep等命令行工具，利用正则表达式在大量文本文件中查找特定内容。 7. **编程语言支持**：熟悉JavaScript、Java、PHP、Python、C#、.NET框架以及MySQL和Oracle数据库管理系统中使用正则表达式的方法。 8. **数据验证**：学习如何创建正则表达式来验证和处理各种格式的数据，如电话号码、邮政编码、社会安全号码、IP地址、URL、电子邮件地址和信用卡号。通过本书的学习，你将能够克服对正则表达式的畏惧，掌握这一强大的工具，并能在实际工作中自如运用。无论是简单的文本匹配还是复杂的文本处理任务，正则表达式都能提供高效的解决方案。书中提供的实例和练习将帮助你巩固所学，真正成为正则表达式的大师。

You need to search a file for some specific text, but only at a specific location

(perhaps at the start of a line or at the end of a sentence).

All these scenarios present unique programming challenges. And all of them can be

solved in just about any language that supports conditional processing and string

manipulation. But how complex a task would the solution become? You would need to

loop through words or characters one at a time, perform all sorts of if statement tests,

track lots of flags so as to know what you had found and what you had not, check for

whitespace and special characters, and more. And you would need to do it all

manually, over and over.

Or you could use regular expressions. Each of the preceding challenges can be solved

using well-crafted statements—highly concise strings containing text and special

instructions—statements that may look like this:

\b[Cc][Aa][Rr]\b

Note

Don’t worry if the previous line does not make sense yet; it will shortly.

How Regular Expressions Are Used

Look at the problem scenarios again and you will notice that they all fall into one of

two types: either information is being located (search) or information is being located

and edited (replace). In fact, at its simplest, that is all that regular expressions are ever

used for: search and replace. Every regular expression either matches text (performing

a search) or matches and replaces text (performing a replace).

RegEx Searches

Regular expressions are used in searches when the text to be searched for is highly

dynamic, as in searching for car in the scenario described earlier. For starters, you

need to locate car or CAR or Car or even CaR; that’s the easy part (many search

tools are capable of performing searches that are not case sensitive). The trickier part is

ensuring that scar, carry, and incarcerate are not matched. Some more

sophisticated editors have Match Only Whole Word options, but many don’t, and you

may not be making this change in a document you are editing. Using a regular

expression for the search, instead of just the text car, solves the problem.

Tip

Want to know what the solution to this one is? You’ve actually seen it already—it is

the sample statement shown previously, \b[Cc][Aa][Rr]\b.

It is worth noting that testing for equality (for example, does this user-specified e-mail

address match this regular expression) is a search operation. The entire user-provided

string is being searched for a match (in contrast to a substring search, which is what

searches usually are).

RegEx Replaces

Regular expression searches are immensely powerful, very useful, and not that

difficult to learn. As such, many of the lessons and examples that you will run into are

matches. However, the real power of regex is seen in replace operations, such as in the

earlier scenario in which you replace URLs with clickable URLs. For starters, this

requires that you be able to locate URLs within text (perhaps searching for strings that

start with http:// or https:// and ending with a period or a comma or

whitespace). Then it also requires that you replace the found URL with two

occurrences of the matched string with embedded HTML so that:

http://www.forta.com/

is replaced with

Click here to view code image

<a href="http://www.forta.com">http://www.forta.com/</a>

Or perhaps the text being located is just an address, and not a fully qualified URL, like

this:

www.forta.com

which would also need to be turned into

Click here to view code image

<a href="http://www.forta.com">http://www.forta.com/</a>

The Search and Replace option in most applications could not handle this type of

replace operation, but this task is incredibly easy using a regular expression.

So What Exactly Is a Regular Expression?

Now that you know what regular expressions are used for, a definition is in order.

Simply put, regular expressions are strings that are used to match and manipulate text.

Regular expressions are created using the regular expression language, a specialized

language designed to do everything that was just discussed and more. Like any

language, regular expressions have a specific syntax and instructions that you must

learn, and that is what this book will teach you.

The regular expression language is not a full programming language. It is usually not

How regular expressions are used and how regular expression functionality is exposed

varies from one application to the next. Some applications have menu options and

dialog boxes used to access regular expressions, whereas programming languages

typically provide functions or classes or objects that expose regex functionality.

Furthermore, not all regular expression implementations are the same. There are often

subtle (and sometimes not so subtle) differences between syntax and features.

Appendix A, “Regular Expressions in Popular Applications and Languages,” provides

usage details and notes for many of the applications and languages that support regular

expressions. Before you proceed to the next lesson, consult that appendix to learn the

specifics pertaining to the application or language that you will be using.

To help you get started quickly, you’ll find links to online regular expression testing

tools on this book’s Web page at

Click here to view code image

http://forta.com/books/0134757068/

These online tools are often the simplest way to experiment with regular expressions.

Before You Get Started

Before you go any further, take note of a couple of important points:

When using regular expressions, you will discover that there are almost always

multiple solutions to any problem. Some may be simpler, some may be faster,

some may be more portable, and some may be more capable. There is rarely a

right or wrong solution when writing regular expressions (as long as your solution

works, of course).

As already stated, differences exist between regex implementations. As much as

possible, the examples and lessons used in this book apply to all major

implementations, and differences or incompatibilities are noted as such.

As with any language, the key to learning regular expressions is practice, practice,

practice.

Note

I strongly suggest that you try each and every example as you work through this

book.

Summary

Regular expressions are one of the most powerful tools available for text manipulation.

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