DTD - 元素

In a DTD, XML elements are declared with a DTD element declaration.在DTD中，XML元素是用DTDA元素的声明方式来声明的。

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Declaring an Element声明元素

In the DTD, XML elements are declared with an element declaration. An element declaration has the following syntax:在DTD中，XML元素是用XML元素的声明方式来声明的。元素的声明方式含有以下句法构造：

<!ELEMENT element-name category>or<!ELEMENT element-name (element-content)>

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Empty elements空元素

Empty elements are declared with the category keyword EMPTY:空元素是用类别关键字EMPTY来声明的：

<!ELEMENT element-name EMPTY>example: <!ELEMENT br EMPTY> XML example: <br />

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Elements with only character data纯字符数据的元素

Elements with only character data are declared with #PCDATA inside parentheses:纯字符数据的元素用圆括号中的#PCDATA来声明：

<!ELEMENT element-name (#PCDATA)>example: <!ELEMENT from (#PCDATA)>

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Elements with any contents以any内容声明的元素

Elements declared with the category keyword ANY, can contain any combination of parsable data:以类别关键字ANY声明的元素能包括任何部分数据的结合体。

<!ELEMENT element-name ANY> example: <!ELEMENT note ANY>

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Elements with children (sequences)以child关键字声明的元素（按次序排列）

Elements with one or more children are defined with the name of the children elements inside parentheses:……包含若干个子类别的元素用圆括号中的子元素的名字来定义。

<!ELEMENT element-name (child-element-name)>or<!ELEMENT element-name (child-element-name,child-element-name,.....)> example: <!ELEMENT note (to,from,heading,body)>

When children are declared in a sequence separated by commas, the children must appear in the same sequence in the document. In a full declaration, the children must also be declared, and the children can also have children. The full declaration of the "note" element will be: 当子元素被逗号依次隔开声明时，子元素必须在文件中以相同的顺序出现。在一个完整的声明中，子元素必须也被声明，同时子元素也能有子元素。"note"元素的完整声明是：

<!ELEMENT note (to,from,heading,body)><!ELEMENT to (#PCDATA)><!ELEMENT from (#PCDATA)><!ELEMENT heading (#PCDATA)><!ELEMENT body (#PCDATA)>

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Declaring only one occurrence of the same element 给相同的元素声明一个发生事件

<!ELEMENT element-name (child-name)> example: <!ELEMENT note (message)>

The example declaration above declares that the child element message must occur once, and only once inside the "note" element.上面的实例声明了子元素信息必须出现一次，并且在"note"元素中只出现一次。

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Declaring minimum one occurrence of the same element声明相同元素的最小发生事件

<!ELEMENT element-name (child-name+)> example: <!ELEMENT note (message+)>

The + sign in the example above declares that the child element message must occur one or more times inside the "note" element.上面实例中的加号声明了子元素信息必须在"note"元素中出现一次或几次。

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Declaring zero or more occurrences of the same element 声明0或更多相同元素的出现事件

<!ELEMENT element-name (child-name*)> example: <!ELEMENT note (message*)>

The * sign in the example above declares that the child element message can occur zero or more times inside the "note" element.在上面实例中的*号声明了子元素信息可以在"note"元素中出现0或更多次数。

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Declaring zero or one occurrences of the same element 声明0或1次相同元素的发生事件

<!ELEMENT element-name (child-name?)> example: <!ELEMENT note (message?)>

The ? sign in the example above declares that the child element message can occur zero or one times inside the "note" element.在上面的实例中的？号声明了子元素信息可以在"note"元素中出现0或1次。

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Declaring either/or content声明either/or内容

example: <!ELEMENT note (to,from,header,(message|body))>

The example above declares that the "note" element must contain a "to" element, a "from" element, a "header" element, and either a "message" or a "body" element.上面的事例声明了"note"元素必须包含一个”TO”元素、一个“form”元素、一个“header”元素以及“message”或“body”元素。

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Declaring mixed content声明混合内容

example: <!ELEMENT note (#PCDATA|to|from|header|message)*>

The example above declares that the "note" element can contain zero or more occurrences of parsed character, "to", "from", "header", or "message" elements.上面的实例声明了"note"元素可以包含0或更多分列字符（“or”、“form”、“header”或“message”元素）。