SGML/XML的文件主要由如下三部分构成:Elements, Attributes and Entities。
Elements(元素)
Describe a document's content and structure. 如:<sect1> </sect1>.
Attributes(元素的属性)
Elements can, but don't necessarily, include one or more attributes, which are additional terms that extend the function or refine the content of a given element.
<sect1 id="idvalue">, id 是 元素 sect1的属性。
Entities(实体)
In the most general terms, entities allow you to assign a name to some chunk of data, and use that name to refer to that data.
general entities and parameter entities:
General Entities(一般实体:包括内部实体和外部实体)
由 & 和 ; 分隔的为实体。包括内部实体(定义在文件内部,引用的文本数据等) 和外部实体(指向文件外部的文件等)。
如: <!ENTITY ora "O'Reilly & Associates"> 为内部实体。 如: "lt; 为指向 < 的内部实体。 <!ENTITY ch01 SYSTEM "ch01.sgm"> 为外部实体。 再入: <!ENTITY tree SYSTEM "tree.eps" NDATA EPS> 为指向外部图形的实体。
Parameter Entities(参数实体)
只用在DTD中的实体声明。是由 % 和 ; 分隔的。在“标记块”中引用,实现标记块是被 忽略还是包含。对于SGML,“标记块”可以出现在DTD和SGML文档中;而对于XML,只出现 在DTD中。
marked sections 中常用的关键字为 INCLUDE 和 IGNORE。
Parameter entities are most frequently used to customize the DTD.
You might use a parameter entity reference in an SGML document in a marked section. Marking sections is a mechanism for indicating that special processing should apply to a particular block of text. Marked sections are introduced by the special sequence <![keyword[ and end with ]]>. In SGML, marked sections can appear in both DTDs and document instances. In XML, they're only allowed in the DTD.
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