split strings on first and last commas

Question

I would like to split strings on the first and last comma. Each string has at least two commas. Below is an example data set and the desired result.

A similar ques

Answer 1

You can use the \K operator which keeps text already matched out of the result and a negative look ahead assertion to do this (well almost, there is an annoying comma at the start of the middle portion which I am yet to get rid of in the strsplit). But I enjoyed this as an exercise in constructing a regex...

x <- '123,34,56,78,90'
strsplit( x , "^[^,]+\\K|,(?=[^,]+$)" , perl = TRUE )
#[[1]]
#[1] "123"       ",34,56,78" "90"

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Explantion:

• ^[^,]+ : from the start of the string match one or more characters that are not a ,
• \\K : but don't include those matched characters in the match
• So the first match is the first comma...
• | : or you can match...
• ,(?=[^,]+$) : a , so long as it is followed by [(?=...)] one or more characters that are not a , until the end of the string ($)...