问题描述
我想用gsub用2个反斜杠替换字符串中每个出现的反斜杠.
I would like to use gsub to replace every occurrence of a backslash in a string with 2 backslashes.
当前，我尝试的是gsub("\\\\", "\\", x).不过，这似乎不起作用.但是，如果我更改表达式以将每个反斜杠替换为"a"，则效果很好.
Currently, what I have I tried is gsub("\\\\", "\\", x). This doesn't seem to work though. However, if I change the expression to instead replace each backslash with "a", it works fine.
> gsub("\\\\", "\\", "\\")
[1] ""
> gsub("\\\\", "a", "\\")
[1] "a"
> gsub("\\\\", "\\\\", "\\")
[1] "\\"
最后一个字符只是一个反斜杠； R只打印2，因为它打印带有反斜杠的转义字符.使用nchar确认长度为1.
The last character is only a single backslash; R just prints 2 because it prints escaped characters with the backslash. Using nchar confirms that the length is 1.
什么原因导致此功能? gsub的第二个参数不是正则表达式，因此字符串文字中具有4个反斜杠的字符应转换为具有2个反斜杠的字符.上面的第一个gsub调用返回空字符串的意义更小.
What causes this functionality? The second argument to gsub isn't a regular expression, so having 4 backslashes in the string literal should be converted to a character with 2 backslashes. It makes even less sense that the first gsub call above returns an empty string.
推荐答案
这是您需要的:
gsub("\\\\", "\\\\\\\\", "\\")
[1] "\\\\"
之所以需要四个反斜杠来表示一个文字反斜杠，是因为"\"既是R字符串中的转义字符，也是正要传递其模式的正则表达式引擎的转义字符.如果您直接与正则表达式引擎对话，则可以使用"\\"表示文字反斜杠.但是为了使R将"\\"传递到正则表达式引擎，您需要键入"\\\\".
The reason that you need four backslashes to represent one literal backslash is that "\" is an escape character in both R strings and for the regex engine to which you're ultimately passing your patterns. If you were talking directly to the regex engine, you'd use "\\" to indicate a literal backslash. But in order to get R to pass "\\" on to the regex engine, you need to type "\\\\".
(如果您只是想将反斜杠加倍，则可能要使用它):
(If you are just wanting to double backslashes, you might want to use this instead):
gsub("\\", "\\\\", "\\", fixed=TRUE)
[1] "\\\\"
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