Home Backend Development XML/RSS Tutorial In-depth understanding of character entities and character data in XML

In-depth understanding of character entities and character data in XML

Mar 10, 2017 pm 07:42 PM

这篇文章主要介绍了XML中的字符实体与字符数据,文中所说的CDATA片段即是指字符数据即不由解析器解析的文本块,需要的朋友可以参考下

字符实体
在我们了解字符实体之前,先了解一下什么是 XML 实体。

W3 联盟 对实体的定义:文档实体作为实体树的根元素和 XML 处理程序的起点。

这意味着实体就是 XML 中的占位符。它们可以声明在文档序言或者DTD中。实体有不同的类型,本章后面会讨论字符实体。

HTML 和 XML 都有一些符号被保留使用,它们不能在 XML 代码中用作内容。例如,< 和 > 符号被用作 XML 标签的开头和结尾。要显示这些特殊字符,就要使用字符实体。

有些特殊的字符或符号还不能直接使用键盘输入。可以使用字符实体来显示这些符号或特殊字符。

字符实体类型
有三种类型的字符实体:

预定义的字符实体。
编码字符实体。
命名字符实体。
预定义的字符实体

引入它们是为了避免使用某些有歧义的符号。例如,当我们把小于(<)或大于(>)符号当尖括号(<>)使用时就会产生歧义。字符实体从根本上来说就是用来界定 XML 标签的。下面是一个来自 XML 规范的预定义字符实体列表。它们可以用来表示某些字符而不带歧义。

和号: &
单引号: '
大于: >
小于: <
双引号: "
数字字符实体

数字引用被用来表示字符实体。数字引用可以是十进制或十六进制格式。由于有数以千记的数字引用,导致很难记住它们。数字引用通过使用字符在 Unicode 字符集中的编码数值来表示。

十进制数字引用基本语法:

4e6f46b2a38cc79fccef890757fa347d

在上述的语法中, 之间的所有内容都会被当做字符数据而不是标记。

CDATA 规则
XML CDATA 需要遵循以下规则:

XML 文档任何位置的 CDATA 都不能包含 "]]>" 字符。
CDATA 片段不可以嵌套。


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