为什么 XML 字符约束是不对称的? [英] Why XML Character constraint is asymmetric?

问题描述

我不清楚 XML 字符验证中不对称背后的逻辑.

Is not clear to me the logic behind the asymmerty in XML characters validation.

例如，后续的 XML 无效(如我所料):

For example the subsequent XMLs are not valid (as i expect) :

<xml>
<value attr="<">my value</value>
</xml>

<xml>
<value attr="attribute">my value is < than</value>
</xml>

但那些 XML 是有效的

But those XMLs are valid

<xml>
<value attr=">">my value</value>
</xml>

<xml>
<value attr="attribute">my value is > than</value>
</xml>

我期待的是任何字符都喜欢<>& 应始终被视为非法.所以我想问一下选择的原因是什么(> 很好，但 <不是).

What i'm expecting is that any characters like <>& should be always considered as illegal. So i would like to ask which are the reason of that choice (> is fine but < is not).

推荐答案

编写语法规则是为了避免解析器必须提前扫描以正确解释字符.

The grammar rules were written to obviate the need for parsers to have to scan ahead to properly interpret characters.

< 和 > 的区别在于解析器在遇到 < 时无法知道它是标签的开始还是一个 LESS THAN 字符而不向前扫描，而当遇到 > 时，解析器根据它的扫描历史(无需提前扫描)知道它是否应该被解释为标签结束或大于字符.

The difference between < and > is that the parser upon encountering < cannot know whether it's the start of a tag or a LESS THAN character without scanning forward, whereas when encountering >, the parser knows based upon its scan history (without having to scan ahead) whether it should be interpreted as end of tag or a GREATER THAN character.

另见:

• 简化的 XML 转义
• Michael Kay 的有用的评论 关于 SGML 兼容性和规则统一性.

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