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In addition to a detailed basic introduction to XML that Web programmers are concerned about
In addition to a detailed basic introduction to XML that Web programmers are concerned about
XML与HTML
首先，XML与HTML类似的，都是一种标记语言。
当初在设计XML时，并非为了将HTML赶下神坛，实际上，XML是为了另外一种目的设计的。
XML旨在传输信息，HTML旨在显示信息
HTML可以有什么标签，不能有什么标签，是被一系列规范约束的；
但在XML中，可以定义自己的标签。
XML有什么用
由于XML是纯文本格式的，因此独立于任何硬件和软件，是一种真正的跨平台数据传输格式。
在XML的基础上，许多其它的技术也得以诞生，比如我们最熟悉的web service，WSDL机制便是基于XML来实现的(也有基于JSON实现的)。
这都得益于XML是一种扩展性非常高的数据传输格式。
语法规则
必须有关闭标签
如<a>，则必须有一个对应的与之配对，当然用<a name=""/>则是一种简略的缩写。大小写敏感
XML文档必须有根元素
属性值必须加引号
-
特殊字符的转义
< - < > - > & - & ' - ' " - "
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元素命名规则
名称可以含字母、数字以及其他的字符
名称不能以数字或者标点符号开始
名称不能以字符 “xml”（或者 XML、Xml）开始
名称不能包含空格
属性
属性必须加引号，如果属性本身有双引号，就用单引号包围它
<a name='steve "aplple" Jobs' />
XML验证
有许多验证方式来验证XML格式是否良好。常用的有以下两种：
XML DTD
XML Schema
XML DTD
合法的 XML 文档是“形式良好”的 XML 文档，同样遵守文档类型定义 (DTD) 的语法规则：
<?xml version="1.0" encoding="ISO-8859-1"?> <!DOCTYPE note SYSTEM "Note.dtd"> <note> <to>George</to> <from>John</from> <heading>Reminder</heading> <body>Don't forget the meeting!</body> </note>
在上例中，DOCTYPE 声明是对外部 DTD 文件的引用。下面的段落展示了这个文件的内容。
<!DOCTYPE note [ <!ELEMENT note (to,from,heading,body)> <!ELEMENT to (#PCDATA)> <!ELEMENT from (#PCDATA)> <!ELEMENT heading (#PCDATA)> <!ELEMENT body (#PCDATA)> ]>
XML Schema
W3C 支持一种基于 XML 的 DTD 代替者，它名为 XML Schema：
<xs:element name="note">
<xs:complexType>
<xs:sequence>
<xs:element name="to" type="xs:string"/>
<xs:element name="from" type="xs:string"/>
<xs:element name="heading" type="xs:string"/>
<xs:element name="body" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>XML命名空间
命名空间的主要目的是解决元素命名冲突的问题。以下两份XML配置文件有命名冲突的问题:
<table> <tr> <td>Apples</td> <td>Bananas</td> </tr> </table>
<table> <name>African Coffee Table</name> <width>80</width> <length>120</length> </table>
使用命名空间来解决冲突后:
<table xmlns="http://www.w3.org/TR/html4/"> <tr> <td>Apples</td> <td>Bananas</td> </tr> </table>
<table xmlns="http://www.w3school.com.cn/furniture"> <name>African Coffee Table</name> <width>80</width> <length>120</length> </table>
命名空间xmlns属性值本身并没有多大含义，只是为了区分命名空间的不同，但实际上xmlns会被开发者用来标识某些资源。
XML CDATA
所有 XML 文档中的文本均会被解析器解析。
只有 CDATA 区段（CDATA section）中的文本会被解析器忽略。
PCDATA
PCDATA 指的是被解析的字符数据（Parsed Character Data）。
XML 解析器通常会解析 XML 文档中所有的文本。
当某个 XML 元素被解析时，其标签之间的文本也会被解析：
<message>此文本也会被解析</message>
解析器之所以这么做是因为 XML 元素可包含其他元素，就像这个例子中，其中的
<name><first>Bill</first><last>Gates</last></name>
而解析器会把它分解为像这样的子元素：
<name>
<first>Bill</first>
<last>Gates</last>
</name>CDATA
术语 CDATA 指的是不应由 XML 解析器进行解析的文本数据（Unparsed Character Data）。
在 XML 元素中，"<" 和 "&" 是非法的。
"<" 会产生错误，因为解析器会把该字符解释为新元素的开始。
"&" 也会产生错误，因为解析器会把该字符解释为字符实体的开始。
某些文本，比如 JavaScript 代码，包含大量 "<" 或 "&" 字符。为了避免错误，可以将脚本代码定义为 CDATA。
CDATA 部分中的所有内容都会被解析器忽略。
CDATA 部分由 " 开始，由 "]]>" 结束：
<script>
<![CDATA[
function matchwo(a,b)
{
if (a < b && a < 0) then
{
return 1;
}
else
{
return 0;
}
}
]]>
</script>在上面的例子中，解析器会忽略 CDATA 部分中的所有内容。
关于 CDATA 部分的注释：
CDATA 部分不能包含字符串 "]]>"。也不允许嵌套的 CDATA 部分。
标记 CDATA 部分结尾的 "]]>" 不能包含空格或折行。
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