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ARROW-12608: [C++][Python][R] Add split_pattern_regex kernel#10354

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ARROW-12608: [C++][Python][R] Add split_pattern_regex kernel#10354
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@lidavidm lidavidm commented May 18, 2021

This adds a split_pattern_regex kernel using RE2.

Caveats:

  • RE2 requires us to wrap the user's regex in a capture group in order to actually get the matched delimiter.
  • Reverse splitting is not implemented - there's not a good way to do this with RE2.
  • In R, strsplit behaves differently - trailing empty splits are no longer dropped:
> df <- tibble(x = c("foo bar"))
> (df %>% mutate(x = strsplit(x, "bar")) %>% collect())$x
[[1]]
[1] "foo "

> (record_batch(df) %>% mutate(x = strsplit(x, "bar")) %>% collect())$x
<list<character>[1]>
[[1]]
[1] "foo " ""

So the behavior here does not exactly match R. Though this was already the case:

> (df %>% mutate(x = strsplit(x, "bar", fixed = TRUE)) %>% collect())$x
[[1]]
[1] "foo "

> (record_batch(df) %>% mutate(x = strsplit(x, "bar", fixed = TRUE)) %>% collect())$x
<list<character>[1]>
[[1]]
[1] "foo " ""

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lidavidm commented May 18, 2021

One question I have is if we might want to still use split_pattern if we detect the pattern is not actually a regex, just so we don't invoke RE2/require an Arrow-with-RE2 build in all cases.

@nealrichardson
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nealrichardson commented May 18, 2021

One question I have is if we might want to still use split_pattern if we detect the pattern is not actually a regex, just so we don't invoke RE2/require an Arrow-with-RE2 build in all cases.

@thisisnic did something similar in #10190, specifically https://github.com/apache/arrow/pull/10190/files#diff-3a791b6bbfdb1605c74f85ca5e9854503ed70ac8b274288919dfbec7ce0cef02R549-R552

@lidavidm
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lidavidm commented May 18, 2021

One question I have is if we might want to still use split_pattern if we detect the pattern is not actually a regex, just so we don't invoke RE2/require an Arrow-with-RE2 build in all cases.

@thisisnic did something similar in #10190, specifically https://github.com/apache/arrow/pull/10190/files#diff-3a791b6bbfdb1605c74f85ca5e9854503ed70ac8b274288919dfbec7ce0cef02R549-R552

Right, and currently this removes that check in favor of always dispatching to the new kernel unless fixed = TRUE. But we could keep that check and only dispatch if fixed = FALSE and the pattern is actually regex-like. That'd let you still use strsplit even if you built Arrow without RE2 in the same cases as before.

It's a very minor point, though - maybe not worth doing.

@nealrichardson
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nealrichardson commented May 18, 2021

Yeah, I thought you were proposing moving that logic into C++ or something. Not sure it's worth doing, if you call a _regex function you must be expecting a build with RE2, and I'd expect that RE2 would optimize non-regex evaluation itself.

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github-actions bot commented May 18, 2021

https://issues.apache.org/jira/browse/ARROW-12608

cyb70289
cyb70289 reviewed May 19, 2021
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cyb70289 approved these changes May 20, 2021
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LGTM, +1

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lidavidm commented May 21, 2021

Thanks for the review, merging now.

@lidavidm lidavidm closed this in e86beb8 May 21, 2021
@asfimport asfimport mentioned this pull request May 21, 2021
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