How does zero-width negative lookahead assertions work in R?

Question

the output of

strsplit('abc dcf', split = '(?=c)', perl = T)

is as expected.

However, the output of

strsplit('abc dcf', split = '(?!c)', perl = T)

is

[[1]]
[1] "a" "b" "c" " " "d" "c" "f"

while my expectation is

[[1]]
[1] "a"  "b"  "c " "d"  "cf"

becasue I thought it wouldn't be splited if the last character of previous chunk matches the char c. Is my understanding of negative lookahead wrong?

@Toto. That seems not the issue. `strsplit('abc dcf', split = '(?!c)', perl = T)` give the same output as `strsplit('abc dcf', split = '(? — mt1022, Feb 15 '17 at 11:25
It does not really matter if it is a positive or negative lookahead. What matters is how `strsplit` deals with zero-length matches. — Wiktor Stribiżew, Feb 15 '17 at 12:13

akrun · Accepted Answer · 2017-02-15T11:39:14.653

1

We can try

strsplit('abc dcf', "(?![c ])\\s*\\b", perl=TRUE)
#[[1]]
#[1] "a"  "b"  "c " "d"  "cf"

edited Feb 15 '17 at 11:39

answered Feb 15 '17 at 11:16

akrun

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HI. could you explain why my code does not work? I am still confused. In addition, I don't need to remove the space. – mt1022 Feb 15 '17 at 11:23
@mt1022 Updated the post by not removing the space – akrun Feb 15 '17 at 11:41

How does zero-width negative lookahead assertions work in R?

1 Answers1