十六进制代码（\x）和unicode（\u）字符有什么区别？ [英] What's the difference between hex code (\x) and unicode (\u) chars?

问题描述

 > \xnn给定十六进制代码（1或2个十六进制数字）的字符
 \ unnnn带给定代码的Unicode字符（1--4十六进制数字）
  

在Unicode字符只有一个或两个数字的情况下，我希望这些字符是相同的。事实上，？Quotes 帮助页面上的一个示例显示：

 \x48\x65\x6c\x6c\x6f\x20\x57\x6f\x72\x6c\x64\x21
 ## [1]你好，世界！ 
\u48\u65\u6c\u6c\u6f\u20\u57\u6f\u72\u6c\u64\u21
 ## [1 ] 你好，世界！ 
  

然而，在Linux下，当试图打印磅符号时，我看到

  cat（\ua3）
 ## $ 
 cat（\xa3）
＃ ＃ 
  

也就是说， \x 十六进制代码无法正确显示。 （这种行为持续存在于我尝试过的任何语言环境中。）在Windows 7下，两个版本都显示一个磅符号。

如果我转换为整数并返回，那么井号正确显示在Linux下。

  cat（intToUtf8（utf8ToInt（\xa3）））
 ##£$ b顺便说一下，这在Windows下不起作用，因为 utf8ToInt（\xa3 ）返回 NA 。
 
 
 
一些 \x 字符在Windows下返回 NA 但在Linux下引发错误。例如：
 
 
  utf8ToInt（\xf0）
 ## utf8ToInt错误（\xf0 ）：无效的UTF-8字符串
  
（\uf0 ）
 
 
这些例子表明 \x 和 \u 字符形式，这似乎是特定于操作系统的，但我无法看到它们如何定义的逻辑。 
 
 
 
这两个字符形式有什么区别？ 
 
解决方案
转义序列 \xNN 将原始字节 NN 插入到字符串中，而 \ uNN 将Unicode代码点 NN 的UTF-8字节插入到UTF-8字符串中：
 > charToRaw（'\xA3'）
 [1] a3 
> charToRaw（'\uA3'）
 [1] c2 a3 
  
这两种类型转义序列不能混合在同一个字符串中： 
 
 
 > '\ua3\xa3'
错误：不允许在字符串中混合Unicode和八进制/十六进制转义
  
这是因为转义序列还定义了字符串的编码。  \uNN 序列显式地将整个字符串的编码设置为UTF-8，而 \xNN 将其保留在默认的未知（又名本地）编码中： 
 
 
 >编码（'\xa3'）
 [1]未知
>编码（'\ua3'）
 [1]UTF-8
  
在打印字符串时变得很重要，因为它们需要转换成适当的输出编码（例如，您的控制台）。具有已定义编码的字符串可以进行适当的转换（请参阅 enc2native ），但那些带有未知编码的字符串原样输出：
 
 
 
 
 
在Linux上，您的控制台可能期望使用UTF-8文本，并且 0xA3 不是有效的UTF-8在Windows上，控制台可能需要Windows-1252文本，而 0xA3 是正确的编码为£，这就是你所看到的。 （当字符串 \uA3 时，将发生从UTF-8到Windows-1252的转换。）
 
 
 
 
 
如果显式设置编码，则适当的转换将发生在Linux上： 
 
 
 > ; s<  - '\xa3'
>编码<  - 'latin1'
> cat（s）
£
  
 
From ?Quotes:

\xnn   character with given hex code (1 or 2 hex digits)  
\unnnn Unicode character with given code (1--4 hex digits)

In the case where the Unicode character has only one or two digits, I would expect these characters to be the same.  In fact, one of the examples on the ?Quotes help page shows:
"\x48\x65\x6c\x6c\x6f\x20\x57\x6f\x72\x6c\x64\x21"
## [1] "Hello World!"
"\u48\u65\u6c\u6c\u6f\u20\u57\u6f\u72\u6c\u64\u21"
## [1] "Hello World!"
However, under Linux, when trying to print a pound sign, I see
cat("\ua3")
## £
cat("\xa3")
## �
That is, the \x hex code fails to display correctly.  (This behaviour persisted with any locale that I tried.)  Under Windows 7 both versions show a pound sign.


If I convert to integer and back then the pound sign displays correctly under Linux.
cat(intToUtf8(utf8ToInt("\xa3")))
## £
Incidentally, this doesn't work under Windows, since utf8ToInt("\xa3") returns NA.

Some \x characters return NA under Windows but throw an error under Linux.  For example:
utf8ToInt("\xf0")
## Error in utf8ToInt("\xf0") : invalid UTF-8 string
("\uf0" is a valid character.)

These examples show that there are some differences between \x and \u forms of characters, which seem to be OS-specific, but I can't see any logic in how they are defined. 

What are the difference between these two character forms?
解决方案
The escape sequence \xNN inserts the raw byte NN into a string, whereas \uNN inserts the UTF-8 bytes for the Unicode code point NN into a UTF-8 string:
> charToRaw('\xA3')
[1] a3
> charToRaw('\uA3')
[1] c2 a3
These two types of escape sequence cannot be mixed in the same string:
> '\ua3\xa3'
Error: mixing Unicode and octal/hex escapes in a string is not allowed
This is because the escape sequences also define the encoding of the string. A \uNN sequence explicitly sets the encoding of the entire string to "UTF-8", whereas \xNN leaves it in the default "unknown" (aka. native) encoding:
> Encoding('\xa3')
[1] "unknown"
> Encoding('\ua3')
[1] "UTF-8"
This becomes important when printing strings, as they need to be converted into the appropriate output encoding (e.g., that of your console). Strings with a defined encoding can be converted appropriately (see enc2native), but those with an "unknown" encoding are simply output as-is:


On Linux, your console is probably expecting UTF-8 text, and as 0xA3 is not a valid UTF-8 sequence, it gives you "�".
On Windows, your console is probably expecting Windows-1252 text, and as 0xA3 is the correct encoding for "£", that's what you see. (When the string is \uA3, a conversion from UTF-8 to Windows-1252 takes place.)


If the encoding is set explicitly, the appropriate conversion will take place on Linux:
> s <- '\xa3'
> Encoding(s) <- 'latin1'
> cat(s)
£


                        
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