diff --git a/stl/inc/regex b/stl/inc/regex
index 2b0e47f68e0..dc5c484cf54 100644
--- a/stl/inc/regex
+++ b/stl/inc/regex
@@ -627,6 +627,11 @@ public:
         return _Compare(_Ptr, _Traits::length(_Ptr));
     }
 
+    void swap(sub_match& _Other) noexcept(_Is_nothrow_swappable<_BidIt>::value) {
+        this->_Mybase::swap(_Other);
+        _STD swap(matched, _Other.matched);
+    }
+
     int _Compare(const value_type* const _Ptr, const _Size_type _Count) const {
         // compare *this to array [_Ptr, _Ptr + _Count)
         const _Mybase _Range(_Effective_range());
diff --git a/tests/std/tests/Dev09_173612_tr1_regex_leak/test.cpp b/tests/std/tests/Dev09_173612_tr1_regex_leak/test.cpp
index 203cbb82438..f74e3eaefcf 100644
--- a/tests/std/tests/Dev09_173612_tr1_regex_leak/test.cpp
+++ b/tests/std/tests/Dev09_173612_tr1_regex_leak/test.cpp
@@ -1,15 +1,19 @@
 // Copyright (c) Microsoft Corporation.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+#include <cassert>
 #include <cstddef>
 #include <cstdio>
 #include <cstdlib>
+#include <cstring>
 #include <locale>
 #include <new>
 #include <regex>
 
 using namespace std;
 
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
 #pragma warning(disable : 28251) // Inconsistent annotation for 'new': this instance has no annotations.
 
 const int N = 1000;
@@ -166,6 +170,31 @@ void test(const char* s) {
     }
 }
 
+// Also test LWG-3204: sub_match::swap only swaps the base class
+void test_lwg3204() {
+    csub_match sm1{};
+    sm1.first   = "hello";
+    sm1.second  = "world";
+    sm1.matched = true;
+
+    csub_match sm2{};
+    sm2.first   = "fluffy";
+    sm2.second  = "cat";
+    sm2.matched = false;
+
+    sm1.swap(sm2);
+
+    assert(strcmp(sm1.first, "fluffy") == 0);
+    assert(strcmp(sm1.second, "cat") == 0);
+    assert(!sm1.matched);
+
+    assert(strcmp(sm2.first, "hello") == 0);
+    assert(strcmp(sm2.second, "world") == 0);
+    assert(sm2.matched);
+
+    STATIC_ASSERT(noexcept(sm1.swap(sm2)));
+}
+
 
 int main() {
     // Perform any locale allocations before we begin the tests.
@@ -207,4 +236,6 @@ int main() {
     test("Huck[[:alpha:]]+");
     test("Tom|Sawyer|Huckleberry|Finn");
     test("Twain");
+
+    test_lwg3204();
 }