R: strsplit on backslash (\)

regex r strsplit
Bert Neef picture Bert Neef · Oct 19, 2015 · Viewed 7.9k times · Source

I am trying to extract the part of the string before the first backslash but I can't seem to get it tot work properly.

I have tried multiple ways of getting it to work, based on the manual page for strsplit and after searching online.

In my actual situation the strings are in a dataframe which I get from a database connection but I can simplify the situation with the following:

> strsplit("BLAAT1\022E:\\BLAAT2\\BLAAT3","\\",fixed=TRUE)
[[1]]
[1] "BLAAT1\022E:" "BLAAT2"      "BLAAT3"  

> strsplit("BLAAT1\022E:\\BLAAT2\\BLAAT3","\\",fixed=FALSE)
Error in strsplit("BLAAT1\022E:\\BLAAT2\\BLAAT3", "\\", fixed = FALSE) : 
  invalid regular expression '\', reason 'Trailing backslash'

> strsplit("BLAAT1\022E:\\BLAAT2\\BLAAT3","\\\\",fixed=TRUE)
[[1]]
[1] "BLAAT1\022E:\\BLAAT2\\BLAAT3"

> strsplit("BLAAT1\022E:\\BLAAT2\\BLAAT3","\\\\",fixed=FALSE)
[[1]]
[1] "BLAAT1\022E:" "BLAAT2"       "BLAAT3"

The expected output would also split on the \ between BLAAT1 and 022E:

Thanks in advance

Answer

Wiktor Stribiżew picture Wiktor Stribiżew · Oct 19, 2015

If you use a regex with strsplit function, a literal backslash can be coded as two literal backslashes (as a literal \ is a special regex metacharacter that is used to form regex escapes, like \d, \w, etc.), but since R string literals support string escape sequences (like "\r" for carriage return, "\n" for a newline char) a literal backslash needs to be defined with a double backslash.

So, "\\" is a literal \, and a regex pattern to match a literal backslash char, being \\, should be coded with 4 backslashes, "\\\\".

Here is a regex that you can use: it splits at \ and a non-printable character:

strsplit("BLAAT1\022E:\\BLAAT2\\BLAAT3","\\\\|[^[:print:]]",fixed=FALSE)
# [1] "BLAAT1" "E:"     "BLAAT2" "BLAAT3"

See IDEONE demo

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