diff --git a/ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java b/ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java
index d8e3cb0f0a..7ca0a6af2b 100644
--- a/ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java
+++ b/ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java
@@ -688,7 +688,15 @@ public IRubyObject matched(ThreadContext context) {
     public IRubyObject matched_size(ThreadContext context) {
         check(context);
         if (!isMatched()) return context.nil;
-        return RubyFixnum.newFixnum(context.runtime, regs.getEnd(0) - regs.getBeg(0));
+
+        int size = str.size();
+        int beg = adjustRegisterPosition(regs.getBeg(0));
+        int end = adjustRegisterPosition(regs.getEnd(0));
+
+        if (beg > size) return context.nil;
+        if (end > size) end = size;
+
+        return RubyFixnum.newFixnum(context.runtime, end - beg);
     }
 
     @JRubyMethod(name = "[]")
diff --git a/ext/strscan/strscan.c b/ext/strscan/strscan.c
index e879a7f7fa..d46e15f7dc 100644
--- a/ext/strscan/strscan.c
+++ b/ext/strscan/strscan.c
@@ -1673,10 +1673,14 @@ static VALUE
 strscan_matched_size(VALUE self)
 {
     struct strscanner *p;
+    long beg, end;
 
     GET_SCANNER(self, p);
     if (! MATCHED_P(p)) return Qnil;
-    return LONG2NUM(p->regs.end[0] - p->regs.beg[0]);
+    beg = adjust_register_position(p, p->regs.beg[0]);
+    if (beg > S_LEN(p)) return Qnil;
+    end = minl(adjust_register_position(p, p->regs.end[0]), S_LEN(p));
+    return LONG2NUM(end - beg);
 }
 
 static int
diff --git a/lib/strscan/truffleruby.rb b/lib/strscan/truffleruby.rb
index 691a951b19..55c1759356 100644
--- a/lib/strscan/truffleruby.rb
+++ b/lib/strscan/truffleruby.rb
@@ -115,7 +115,15 @@ def rest?
 
   def matched? = !Primitive.nil?(@last_match)
 
-  def matched = @last_match&.to_s
+  def matched
+    return unless @last_match
+
+    beg = Primitive.match_data_byte_begin(@last_match, 0)
+    return if beg > @string.bytesize
+
+    fin = [Primitive.match_data_byte_end(@last_match, 0), @string.bytesize].min
+    @string.byteslice(beg, fin - beg)
+  end
 
   def [](group)
     raise TypeError, 'no implicit conversion of Range into Integer' if Primitive.is_a?(group, Range)
@@ -151,7 +159,9 @@ def named_captures = @last_match&.named_captures || {}
 
   def matched_size
     if @last_match
-      Primitive.match_data_byte_end(@last_match, 0) - Primitive.match_data_byte_begin(@last_match, 0)
+      beg = Primitive.match_data_byte_begin(@last_match, 0)
+      return if beg > @string.bytesize
+      [Primitive.match_data_byte_end(@last_match, 0), @string.bytesize].min - beg
     end
   end
 
diff --git a/test/strscan/test_stringscanner.rb b/test/strscan/test_stringscanner.rb
index 79784b59f5..8bf24c7301 100644
--- a/test/strscan/test_stringscanner.rb
+++ b/test/strscan/test_stringscanner.rb
@@ -738,6 +738,27 @@ def test_matched_size
     assert_nil(s.matched_size)
   end
 
+  def test_matched_size_when_shrunk
+    # matched_size must agree with matched, which extract_range clamps to the
+    # current length of the stored string.
+    s = create_string_scanner(+"before 29 after")
+    s.skip_until(" ")
+    assert_equal("29", s.scan(/\d+/))
+    assert_equal(2, s.matched_size)
+
+    s.string.replace("before 2")
+    assert_equal("2", s.matched)
+    assert_equal(1, s.matched_size)
+
+    s.string.replace("before ")
+    assert_equal("", s.matched)
+    assert_equal(0, s.matched_size)
+
+    s.string.replace("before")
+    assert_nil(s.matched)
+    assert_nil(s.matched_size)
+  end
+
   def test_empty_encoding_utf8
     ss = create_string_scanner('')
     assert_equal(Encoding::UTF_8, ss.rest.encoding)