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8.2 正则表达式
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8.2 正则表达式
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<div style='float:left;width:49%'>·<a href='100165353.htm'>8.1 System.String类</a></div><div style='float:right;width:49%'>·<a href='100165354.htm'>8.1.1 创建字符串</a></div><div style='float:left;width:49%'>·<a href='100165355.htm'>8.1.2 格式化字符串</a></div><div style='float:right;width:49%'>·<a href='100165357.htm'>8.3 小结</a></div><div style='float:left;width:49%'>·<a href='100165358.htm'>9.1 对象组</a></div><div style='float:right;width:49%'>·<a href='100165359.htm'>9.1.1 数组列表</a></div></div>
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<div id="text"> <link href="css.css" rel="stylesheet" type="text/css" /><h3 style="MARGIN-TOP: 11.4pt; MARGIN-LEFT: 0cm; MARGIN-RIGHT: 0cm; FTEL: 11.4pt"><a ftel="_Toc507815067"><span lang="EN-US">8.2 </span></a><span style="FONT-FAMILY: 楷体_GB2312">正则表达式</span></h3>
<p class="MsoNormal"><a ftel="regularexpressions"><span style="FONT-FAMILY: 宋体">正则表达式在各种程序中都有着难以置信的作用，但并不是所有的开发人员都知道这一点。正则表达式被当作一种有特定功能的小型编程语言：在大的字符串表达式中定位一个子字符串。它不是一种新技术，最初它是在</span><span lang="EN-US">UNIX</span></a><span style="FONT-FAMILY: 宋体">环境中开发的，与</span><span lang="EN-US">Perl</span><span style="FONT-FAMILY: 宋体">一起使用得比较多。</span><span lang="EN-US">Microsoft</span><span style="FONT-FAMILY: 宋体">把它移植到</span><span lang="EN-US">Windows</span><span style="FONT-FAMILY: 宋体">中，到目前为止在脚本语言中用得比较多。但</span><span lang="EN-US">System.Text.Regular- Expressions</span><span style="FONT-FAMILY: 宋体">命名空间中的许多</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">类都支持正则表达式。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">许多您都不太熟悉正则表达式语言，所以本节将主要解释正则表达式和相关的</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">类。如果您很熟悉正则表达式，就可以跳过本节，学习</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">基类的引用。注意，</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">正则表达式引擎是为兼容</span><span lang="EN-US">Perl 5</span><span style="FONT-FAMILY: 宋体">的正则表达式设计的，但有一些新特性。</span></p>
<h3 style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 0cm; MARGIN-RIGHT: 0cm; FTEL: 8.15pt"><a ftel="_Toc507815068"><span lang="EN-US">8.2.1 </span></a><span style="FONT-FAMILY: 黑体">正则表达式</span><span style="FONT-FAMILY: 黑体">概述</span></h3>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">正则表达式语言是一种专门用于字符串处理的语言。它包含两个功能：</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">一组用于标识字符类型的转义代码。您可能很熟悉</span><span lang="EN-US">DOS</span><span style="FONT-FAMILY: 宋体">表达式中的</span><span lang="EN-US">*</span><span style="FONT-FAMILY: 宋体">字符表示任意子字符串</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">例如，</span><span lang="EN-US">DOS</span><span style="FONT-FAMILY: 宋体">命令</span><span lang="EN-US">Dir Re*</span><span style="FONT-FAMILY: 宋体">会列出所有名称以</span><span lang="EN-US">Re</span><span style="FONT-FAMILY: 宋体">开头的文件</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。正则表达式使用与</span><span lang="EN-US">*</span><span style="FONT-FAMILY: 宋体">类似的许多序列来表示“任意一个字符”、“一个单词”、“一个可选的字符”等。</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">一个系统。在搜索操作中，它把子字符串和中间结果的各个部分组合起来。</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">使用正则表达式，可以对字符串执行许多复杂而高级的操作，例如：</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">区分</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">可以是标记或删除</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">字符串中所有重复的单词，例如，把</span><span lang="EN-US">The computer books books</span><span style="FONT-FAMILY: 宋体">转换为</span><span lang="EN-US">The computer books</span><span style="FONT-FAMILY: 宋体">。</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">把所有单词都转换为标题格式，例如把</span><span lang="EN-US">this is a Title</span><span style="FONT-FAMILY: 宋体">转换为</span><span lang="EN-US">This Is A Title</span><span style="FONT-FAMILY: 宋体">。</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">把长于</span><span lang="EN-US">3</span><span style="FONT-FAMILY: 宋体">个字符的所有单词都转换为标题格式，例如把</span><span lang="EN-US">this is a Title</span><span style="FONT-FAMILY: 宋体">转换为</span><span lang="EN-US">This is a Title</span><span style="FONT-FAMILY: 宋体">。</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">确保句子有正确的大写形式。</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">区分</span><span lang="EN-US">URI</span><span style="FONT-FAMILY: 宋体">的各个元素</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">例如</span><span class="screentext-PRODUCTION"><span lang="EN-US" style="FONT-FAMILY: 'Times New Roman'; LETTER-SPACING: 0pt">http://www.wrox.com</span></span><span class="screentext-PRODUCTION"><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0pt">，提取出协议、计算机名、文件名等</span></span><span class="screentext-PRODUCTION"><span lang="EN-US" style="FONT-FAMILY: 'Times New Roman'; LETTER-SPACING: 0pt">)</span></span><span class="screentext-PRODUCTION"><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0pt">。</span></span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">当然，这些都是可以在</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">中用</span><span class="codeintext-PRODUCTION"><span lang="EN-US" style="FONT-FAMILY: 'Times New Roman'">System.String</span></span><span lang="EN-US"> </span><span style="FONT-FAMILY: 宋体">和</span> <span class="codeintext-PRODUCTION"><span lang="EN-US" style="FONT-FAMILY: 'Times New Roman'">System.Text.StringBuilder</span></span><span class="codeintext-PRODUCTION"><span style="FONT-FAMILY: 宋体">执行的任务。但是，在一些情况下，还需要编写相当多的</span></span><span class="codeintext-PRODUCTION"><span lang="EN-US" style="FONT-FAMILY: 'Times New Roman'">C#</span></span><span class="codeintext-PRODUCTION"><span style="FONT-FAMILY: 宋体">代码。如果使用正则表达式，这些代码一般可以压缩为几行代码。实际上，是实例化了一个对象</span></span><span class="codeintext-PRODUCTION"><span lang="EN-US" style="FONT-FAMILY: 'Times New Roman'">System.Text.RegularExpressions.RegEx(</span></span><span class="codeintext-PRODUCTION"><span style="FONT-FAMILY: 宋体">甚至更简单：调用静态的</span></span><span class="codeintext-PRODUCTION"><span lang="EN-US" style="FONT-FAMILY: 'Times New Roman'">RegEx()</span></span><span class="codeintext-PRODUCTION"><span style="FONT-FAMILY: 宋体">方法</span></span><span class="codeintext-PRODUCTION"><span lang="EN-US" style="FONT-FAMILY: 'Times New Roman'">)</span></span><span class="codeintext-PRODUCTION"><span style="FONT-FAMILY: 宋体">，给它传送要处理的字符串和一个正则表达式</span></span><span class="codeintext-PRODUCTION"><span lang="EN-US" style="FONT-FAMILY: 'Times New Roman'">(</span></span><span class="codeintext-PRODUCTION"><span style="FONT-FAMILY: 宋体">这是一个字符串，包含用正则表达式语言编写的指令</span></span><span class="codeintext-PRODUCTION"><span lang="EN-US" style="FONT-FAMILY: 'Times New Roman'">)</span></span><span class="codeintext-PRODUCTION"><span style="FONT-FAMILY: 宋体">，就可以了。</span></span></p>
<p class="MsoNormal"><span class="codeintext-PRODUCTION"><span style="FONT-FAMILY: 宋体">正则表达式字符串初看起来像是一般的字符串，但其中包含了转义序列和有特定含义的其他字符。例如，</span></span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.2pt">序列</span><span lang="EN-US">\b</span><span style="FONT-FAMILY: 宋体">表示一个字的开头和结尾</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">字的边界</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">，如果要表示正在查找以字符</span><span lang="EN-US">th</span><span style="FONT-FAMILY: 宋体">开头的字，就可以编写正则表达式</span><span lang="EN-US">\bth(</span><span style="FONT-FAMILY: 宋体">即序列字边界是</span><span lang="EN-US">– t – h)</span><span style="FONT-FAMILY: 宋体">。如果要搜索所有以</span><span lang="EN-US">th</span><span style="FONT-FAMILY: 宋体">结尾的字，就可以编写</span><span lang="EN-US">th\b(</span><span style="FONT-FAMILY: 宋体">序列</span><span lang="EN-US">t – h–</span><span style="FONT-FAMILY: 宋体">字边界</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。但是，正则表达式要比这复杂得多，例如，可以在搜索操作中找到存储部分文本的工具性程序。本节仅介绍正则表达式的功能。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">假定应用程序需要把</span><span lang="EN-US">US</span><span style="FONT-FAMILY: 宋体">电话号码转换为国际格式。在美国，电话号码的格式为</span><span lang="EN-US">314-123-1234</span><span style="FONT-FAMILY: 宋体">，常常写作</span><span lang="EN-US">(314) 123-1234</span><span style="FONT-FAMILY: 宋体">。在把这个国家格式转换为国际格式时，必须在电话号码的前面加上</span><span lang="EN-US">+1(</span><span style="FONT-FAMILY: 宋体">美国的国家代码</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">，并给区域代码加上括号：</span><span lang="EN-US">+1(314) 123-1234</span><span style="FONT-FAMILY: 宋体">。在查找和替换时，这并不复杂，但如果要使用字符串类完成这个转换，就需要编写一些代码</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">这表示，必须使用</span><span lang="EN-US">System.String</span><span style="FONT-FAMILY: 宋体">上的方法来编写代码</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">，而正则表达式语言可以构造一个短的字符串来表达上述含义。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">所以，本节只有一个非常简单的示例，我们只考虑如何查找字符串中的某些子字符串，无须考虑如何修改它们。</span></p>
<h3 style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 0cm; MARGIN-RIGHT: 0cm; FTEL: 8.15pt"><a ftel="_Toc507815069"><span lang="EN-US">8.2.2 RegularExpressionsPlayaround</span></a><span style="FONT-FAMILY: 黑体">示例</span></h3>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">下面将开发一个小示例，执行并显示一些搜索的结果，说明正则表达式的一些特性，以及如何在</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">中使用</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">正则表达式引擎。这个示例文档中使用的文本是引自另一本有关</span><span lang="EN-US">XML</span><span style="FONT-FAMILY: 宋体">的</span><span lang="EN-US">Wrox Press</span><span style="FONT-FAMILY: 宋体">书</span><span lang="EN-US">(ASP.NET 1.1</span><span style="FONT-FAMILY: 宋体">高级编程，清华大学出版社翻译出版</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">：</span></p>
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