Bug #17118: String#index and #rindex 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 pattern Regexp given to String#index and String#rindex contain a /\K/ (lookbehind) operator, these methods return the position where the beginning of the lookbehind pattern matches, while they are expected to return the position where the \K matches.

# expected -- index of "c" is returned
"abcdbce".index(/b\Kc/)  # => 2
"abcdbce".rindex(/b\Kc/)  # => 5

# actual -- index of "b" is returned
"abcdbce".index(/b\Kc/)  # => 1
"abcdbce".rindex(/b\Kc/)  # => 4

[hanazuki (Kasumi Hanazuki)]

Patch: https://github.com/ruby/ruby/pull/3414

[hanazuki (Kasumi Hanazuki)]

• Status changed from Open to Closed

Applied in changeset git|014a4fda54cb6897ed54ea9c44376db3459fc46e.

---

rb_str_{index,rindex}_m: Handle /\K/ in pattern

When the pattern Regexp given to String#index and String#rindex
contain a /\K/ (lookbehind) operator, these methods return the
position where the beginning of the lookbehind pattern matches, while
they are expected to return the position where the \K matches.

# without patch
"abcdbce".index(/b\Kc/)  # => 1
"abcdbce".rindex(/b\Kc/)  # => 4

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

# with patch
"abcdbce".index(/b\Kc/)  # => 2
"abcdbce".rindex(/b\Kc/)  # => 5

Fixes [Bug #17118]