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]

1 files changed, 4 insertions, 0 deletions

diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index c4c7d55f00..308ba80373 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -2603,6 +2603,8 @@ CODE
     assert_equal("hello", hello, bug)
 
     assert_equal(["", "", "foo"], "foo".partition(/^=*/))
+
+    assert_equal([S("ab"), S("c"), S("dbce")], S("abcdbce").partition(/b\Kc/))
   end
 
   def test_rpartition
@@ -2627,6 +2629,8 @@ CODE
     hello = "hello"
     hello.rpartition("hi").map(&:upcase!)
     assert_equal("hello", hello, bug)
+
+    assert_equal([S("abcdb"), S("c"), S("e")], S("abcdbce").rpartition(/b\Kc/))
   end
 
   def test_setter