Bug #17119: String#partition and #rpartition return wrong result for Regexp patterns containing /\K/

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.8.0dev (2020-08-13T01:14:20Z master 69b5241c36) [x86_64-linux]
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN

[hanazuki (Kasumi Hanazuki)]

When the Regexp pattern given to String#partition and String#rpartition contain a /\K/ (lookbehind) operator, the methods return strings sliced at incorrect positions.

# expected -- partitioned at "c" following "b"
"abcdbce".partition(/b\Kc/)  # => ["ab", "c", "dbce"]
"abcdbce".rpartition(/b\Kc/)  # => ["abcdb", "c", "e"]

# actual -- results are not partitions
"abcdbce".partition(/b\Kc/)  # => ["a", "c", "cdbce"]
"abcdbce".rpartition(/b\Kc/)  # => ["abcd", "c", "ce"]

[hanazuki (Kasumi Hanazuki)]

Patch: https://github.com/ruby/ruby/pull/3413

[hanazuki (Kasumi Hanazuki)]

• Status changed from Open to Closed

Applied in changeset git|5d71eed1a7f0a70db013de59cd7e95bdca0d5c0e.

---

rb_str_{partition,rpartition}_m: Handle /\K/ in pattern

When the pattern given to String#partition and String#rpartition
contain a /\K/ (lookbehind) operator, the methods return strings
sliced at incorrect positions.

# without patch
"abcdbce".partition(/b\Kc/)  # => ["a", "c", "cdbce"]
"abcdbce".rpartition(/b\Kc/)  # => ["abcd", "c", "ce"]

This patch fixes the problem by using BEG(0) instead of the return
value of rb_reg_search.

# with patch
"abcdbce".partition(/b\Kc/)  # => ["ab", "c", "dbce"]
"abcdbce".rpartition(/b\Kc/)  # => ["abcdb", "c", "e"]

As a side-effect this patch makes String#partition 2x faster when the
pattern is a costly Regexp by performing Regexp search only once,
which was unexpectedly done twice in the original implementation.

Fixes [Bug #17119]