R - gsub replacing backslashes

Question

I would like to use gsub to replace every occurrence of a backslash in a string with 2 backslashes.

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] "\\"

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.

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.

Answer 1

Here's what you need:

gsub("\\\\", "\\\\\\\\", "\\")
[1] "\\\\"

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 "\\\\".

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(If you are just wanting to double backslashes, you might want to use this instead):

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