From d3a82fa1a8df9d46078c6f8e440feb3674757089 Mon Sep 17 00:00:00 2001
From: Sutou Kouhei <kou@clear-code.com>
Date: Tue, 6 Oct 2026 13:46:14 +0900
Subject: [PATCH] Reset the number of registers after string pattern match

`set_registers()` used `onig_region_set()` that doesn't change
`num_regs` when the region is already allocated. So
`StringScanner#size` and `#captures` after a string pattern match
reported the number of registers of the previous `Regexp` match:

```ruby
s = StringScanner.new("abc")
s.scan(/(a)(b)/)
s.scan("c")
s.size     # => 3 (should be 1)
s.captures # => [nil, nil] (should be [])
```

Use `onig_region_resize()` like `adjust_registers_to_matched()` does.
This is consistent with the JRuby and TruffleRuby implementations.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 ext/strscan/strscan.c              |  2 +-
 test/strscan/test_stringscanner.rb | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/ext/strscan/strscan.c b/ext/strscan/strscan.c
index 42a1f7f1ad..dd7caf91b0 100644
--- a/ext/strscan/strscan.c
+++ b/ext/strscan/strscan.c
@@ -634,7 +634,7 @@ set_registers(struct strscanner *p, size_t pos, size_t length)
     const int at = 0;
     OnigRegion *regs = &(p->regs);
     onig_region_clear(regs);
-    if (onig_region_set(regs, at, 0, 0)) return;
+    if (onig_region_resize(regs, at + 1) != 0) return;
     if (p->fixed_anchor_p) {
         regs->beg[at] = pos + p->curr;
         regs->end[at] = pos + p->curr + length;
diff --git a/test/strscan/test_stringscanner.rb b/test/strscan/test_stringscanner.rb
index 8530be3893..87b429b29b 100644
--- a/test/strscan/test_stringscanner.rb
+++ b/test/strscan/test_stringscanner.rb
@@ -1025,6 +1025,27 @@ def test_size
     assert_equal(4, s.size)
   end
 
+  def test_size_after_string_match
+    s = create_string_scanner("abc")
+    s.scan(/(a)(b)/)
+    s.scan("c")
+    assert_equal([1, []], [s.size, s.captures])
+  end
+
+  def test_size_after_getch
+    s = create_string_scanner("abc")
+    s.scan(/(a)(b)/)
+    s.getch
+    assert_equal([1, []], [s.size, s.captures])
+  end
+
+  def test_size_after_get_byte
+    s = create_string_scanner("abc")
+    s.scan(/(a)(b)/)
+    s.get_byte
+    assert_equal([1, []], [s.size, s.captures])
+  end
+
   def test_captures
     s = create_string_scanner("Timestamp: Fri Dec 12 1975 14:39")
     s.scan("Timestamp: ")