MichaelChirico
commented
4 years ago I noted that: Not all zero-length look-behind patterns show this problem. E.g.,
strsplit(split="(?<=[[:punct:]])", "One, two; three!", perl=TRUE)[[1]]
[1] "One," " two;" " three!"
However, if I expand that pattern to include the zero-length match at the beginning of the string the problem appears again:
strsplit(split="(?<=[[:punct:]])|^", "One, two; three!", perl=TRUE)[[1]]
[1] "O" "n" "e" "," " " "t" "w" "o" ";" " " "t" "h" "r" "e" "e" "!"
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- Comment author - Bill Dunlap
- Timestamp - 2016-03-03 04:08:23 UTC
The perl regex "[[:<:]]" makes zero-length match at the beginning of a word ("[[:>]]" means end-of-word). It acts properly in gregexpr but not in strsplit:
gregexpr("[[:<:]]", "One, two; three!", perl=TRUE)[[1]]
[1] 1 6 11
attr(,"match.length")
[1] 0 0 0
attr(,"useBytes")
[1] TRUE
strsplit(split="[[:<:]]", "One, two; three!", perl=TRUE)[[1]] # [1] "O" "n" "e" ", " "t" "w" "o" "; " "t" "h" "r" "e" "e" "!" # Expect c("One, ", "two; ", "three!"), breaks before chars 1, 6, and 11
strsplit does act as expected for the zero-length look-ahead pattern "[[:>:]]":
gregexpr("[[:>:]]", "One, two; three!", perl=TRUE)[[1]]
[1] 4 9 16
attr(,"match.length")
[1] 0 0 0
attr(,"useBytes")
[1] TRUE
strsplit(split="[[:>:]]", "One, two; three!", perl=TRUE)[[1]]
[1] "One" ", two" "; three" "!"
Not all zero-length look-behind patterns show this problem. E.g.,
strsplit(split="(?<=[[:punct:]])", "One, two; three!", perl=TRUE)[[1]]
[1] "One," " two;" " three!"
It may be possible that strsplit is not using the startoffset argument to pcre_exec
pcre/pcre/doc/html/pcreapi.html A non-zero starting offset is useful when searching for another match in the same subject by calling pcre_exec() again after a previous success. Setting startoffset differs from just passing over a shortened string and setting PCRE_NOTBOL in the case of a pattern that begins with any kind of lookbehind.
or it could be something else.
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