Bug #18415: String#rpartition is not sufficiently greedy compared to String#partition

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 3.1.0preview1 (2021-11-09 master 5a3b2e6141) [x86_64-darwin21]
• Backport: 2.6: UNKNOWN, 2.7: REQUIRED, 3.0: DONE

[jdashton (J Daniel Ashton)]

3.1.0-preview1 :043 > "...999...".partition /\d+/
 => ["...", "999", "..."] 
3.1.0-preview1 :044 > "...999...".rpartition /\d+/
 => ["...99", "9", "..."] 

Using the regex /\d+/, partition gives you all consecutive digits. However, rpartition gives only the last digit.

[sawa (Tsuyoshi Sawada)]

I think the actual result is expected. It is partition's responsibility to match the left-most position possible, and it is +'s responsibility to match as long as possible from that position. Likewise, it is rpartition's responsibility to match the right-most position possible, and it is +'s responsibility to match as long as possible from that position. ["...99", "9", "..."] matches at index 5, which is more right than the index given by partitioning into ["...", "999", "..."], which is 3. And since matching starts at index 5, there is no extending of the match to index 6, which is ..

If you want to match 999 with rpartition, you need to do something like:

"...999...".rpartition(/(?<!\d)\d+/)
# => ["...", "999", "..."]

[nobu (Nobuyoshi Nakada)]

• Backport changed from 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN to 2.6: UNKNOWN, 2.7: REQUIRED, 3.0: REQUIRED

I agree that this is intentional and a documentation matter.

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed

Applied in changeset git|e2ec97c4b823a0b2e0c31e7a6d77b1dcdc0dfada.

---

[DOC] How to get the longest last match [Bug #18415]

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 2.6: UNKNOWN, 2.7: REQUIRED, 3.0: REQUIRED to 2.6: UNKNOWN, 2.7: REQUIRED, 3.0: DONE

ruby_3_0 4b1cee1431b44e923611c65a8ec5cc61d4025641 merged revision(s) e2ec97c4b823a0b2e0c31e7a6d77b1dcdc0dfada.