R:如何用*单个*反斜杠和空格('\')替换字符串中的空格('') [英] R: How to replace space (' ') in string with a *single* backslash and space ('\ ')

问题描述

我已经搜索了很多次，但在这里或其他地方都没有找到答案.我想用 '\' 替换包含文件名的变量中的每个空格 ' '.(一个用例可能是 shell 命令，空格被转义，所以每个文件名不会显示为参数列表.)我已经浏览了 StackOverflow 问题 如何替换 R 中的单个反斜杠"，并发现许多组合确实如宣传的那样工作:

I've searched many times, and haven't found the answer here or elsewhere. I want to replace each space ' ' in variables containing file names with a '\ '. (A use case could be for shell commands, with the spaces escaped, so each file name doesn't appear as a list of arguments.) I have looked through the StackOverflow question "how to replace single backslash in R", and find that many combinations do work as advertised:

> gsub(" ", "\\\\", "a b")
[1] "a\\b"

> gsub(" ", "\\ ", "a b", fixed = TRUE)
[1] "a\\ b"

但是用单斜线版本试试这些，R 会忽略它:

but try these with a single-slash version, and R ignores it:

> gsub(" ", "\\ ", "a b")
[1] "a b"

> gsub(" ", "\ ", "a b", fixed = TRUE)
[1] "a b"

对于相反方向的情况——从字符串中删除斜线，它适用于两个:

For the case going in the opposite direction — removing slashes from a string, it works for two:

> gsub("\\\\", " ", "a\\b")
[1] "a b"

> gsub("\\", " ", "a\\b", fixed = TRUE)
[1] "a b"

然而，对于单斜线，R 中的一些内部反常使我什至无法尝试删除它们:

However, for single slashes some inner perversity in R prevents me from even attempting to remove them:

> gsub("\\", " ", "a\\b")
Error in gsub("\\", " ", "a\\b") : 
  invalid regular expression '\', reason 'Trailing backslash'

> gsub("\", " ", "a\b", fixed = TRUE)
Error: unexpected string constant in "gsub("\", " ", ""

无效的正则表达式"告诉我们一些事情，但我不明白是什么.(还要注意 perl = True 选项没有帮助.)

The 'invalid regular expression' is telling us something, but I don't see what. (Note too that the perl = True option does not help.)

即使有三个反斜杠，R 也没有注意到一个:

Even with three back slashes R fails to notice even one:

> gsub(" ", "\\\ ", "a b")
[1] "a b"

模式也延伸了！即使是两个工作的倍数:

The patter extends too! Even multiples of two work:

> gsub(" ", "\\\\\\\\", "a b")
[1] "a\\\\b"

但不是奇数倍数(应该得到 '\\\':

but not odd multiples (should get '\\\ ':

> gsub(" ", "\\\\\\ ", "a b")
[1] "a\\ b"

> gsub(" ", "\\\ ", "a b", fixed = TRUE)
[1] "a\\ b"

(我希望有 3 个斜杠，而不是两个.)

(I would expect 3 slashes, not two.)

我的两个问题是:

• 如何实现将 ' ' 替换为 '\ ' 的目标?
• 为什么替换的奇数斜线变体失败，而偶数斜线替换有效?

对于 shell 命令，一个简单的解决方法是引用文件名，但我的部分兴趣只是想了解 R 的正则表达式引擎发生了什么.

For shell commands a simple work-around is to quote the file names, but part of my interest is just wanting to understand what is going on with R's regex engine.

推荐答案

准备好面对面，因为:

> gsub(" ", "\\\ ", "a b", fixed = TRUE)
[1] "a\\ b"

实际上正在工作.

您看到的两个反斜杠只是 R 控制台显示单个反斜杠的方式，在打印到屏幕时会对其进行转义.

The two backslashes you see are just the R console's way of displaying a single backslash, which is escaped when printed to the screen.

要确认用单个反斜杠替换确实有效，请尝试将输出写入文本文件并检查自己:

To confirm the replacement with a single backslash is indeed working, try writing the output to a text file and inspect yourself:

f <- file("C:\\output.txt")
writeLines(gsub(" ", "\\", "a b", fixed = TRUE), f)
close(f)

在 output.txt 中，您应该看到以下内容:

In output.txt you should see the following:

a\b

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