string split on last comma in R

Question

I'm not new to R but I am relatively new to regular expressions.

A similar question can be found in here, but it asks to split on the first comma rather than the last one.

As an example, if I use

> lastcomma_strsplit("UK, USA, Germany", ", ")
[[1]]
[1] "UK"      "USA"     "Germany"

I want to get

[[1]]
[1] "UK, USA"     "Germany"

And if I use

> lastcomma_strsplit("London, Washington, D.C., Berlin", ", ")
[[1]]
[1] "London"     "Washington" "D.C."       "Berlin"

I want to get

[[1]]
[1] "London, Washington, D.C."       "Berlin"

One viable way I think is to replace the last comma by something else such as

$, #, *, ...

then use

strsplit()

to split the string by the one you replaced (Make sure it is unique!), but I'm more happy if you can deal with the problem using some built in function directly.

So how can I do that?

Tyler Rinker · Accepted Answer · 2014-07-24T16:20:15.197

20

Here's one approach:

strsplit("UK, USA, Germany", ",(?=[^,]+$)", perl=TRUE)

## [[1]]
## [1] "UK, USA" " Germany"

You may want:

strsplit("UK, USA, Germany", ",\\s*(?=[^,]+$)", perl=TRUE)

## [[1]]
## [1] "UK, USA" "Germany"

As it will match if there is no space after the comma:

strsplit(c("UK, USA, Germany", "UK, USA,Germany"), ",\\s*(?=[^,]+$)", perl=TRUE)

## [[1]]
## [1] "UK, USA" "Germany"
## 
## [[2]]
## [1] "UK, USA" "Germany"

edited Jul 24 '14 at 16:20

answered Jul 24 '14 at 16:14

Tyler Rinker

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Thanks, this is OK, but you didn't take the second comma away... it is much better to remove the second comma. – Jiqing Huang Jul 24 '14 at 16:17
Thank you very much, mate! Any suggestion or websites to learn basics of regular expression in R? – Jiqing Huang Jul 24 '14 at 16:22
1

Two places: I pick up a lot here on SO and http://www.regular-expressions.info/rlanguage.html – Tyler Rinker Jul 24 '14 at 16:23

score 8 · Answer 2 · edited Oct 13 '16 at 13:46

You can use stri_split function from stringi package

x <- "USA,UK,Poland"
stri_split_fixed(x,",") # standard split by comma
[[1]]
[1] "USA"    "UK"     "Poland"

stri_split_fixed(x,",",n = 2) # set the max number of elements
[[1]]
[1] "USA"       "UK,Poland"

Unfortunately there is no parameter to change the starting point for splitting (from begin/end) but we can handle this another way - using stri_reverse

stri_split_fixed(stri_reverse(x),",",n = 2) #reverse
[[1]]
[1] "dnaloP" "KU,ASU"

stri_reverse(stri_split_fixed(stri_reverse(x),",",n = 2)[[1]]) #reverse back
[1] "Poland" "USA,UK"
stri_reverse(stri_split_fixed(stri_reverse(x),",",n = 2)[[1]])[2:1] #and again :)
[1] "USA,UK" "Poland"

string split on last comma in R

2 Answers2

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