From dc1a84e65b5fd4c4625a309fd11058d5c9eb2e3c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Julian=20M=C3=BCller?= <julian.mueller@uni-konstanz.de>
Date: Fri, 28 Mar 2025 18:16:42 +0100
Subject: [PATCH 1/2] `<regex>`: Fix word bounds `\b` and `\B` when applied to
 the empty string

---
 stl/inc/regex                                  | 2 +-
 tests/std/tests/VSO_0000000_regex_use/test.cpp | 9 ++++++++-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/stl/inc/regex b/stl/inc/regex
index 17fd2ee72c5..e372d6119f1 100644
--- a/stl/inc/regex
+++ b/stl/inc/regex
@@ -3497,7 +3497,7 @@ bool _Matcher<_BidIt, _Elem, _RxTraits, _It>::_Is_wbound() const {
         }
     } else { // --_Cur is not valid
         if (_Tgt_state._Cur == _End) {
-            return (_Mflags & (regex_constants::match_not_bow | regex_constants::match_not_eow)) == 0;
+            return false;
         } else {
             return (_Mflags & regex_constants::match_not_bow) == 0 && _Is_word(*_Tgt_state._Cur);
         }
diff --git a/tests/std/tests/VSO_0000000_regex_use/test.cpp b/tests/std/tests/VSO_0000000_regex_use/test.cpp
index ba9c575940a..7ef92d2f0fb 100644
--- a/tests/std/tests/VSO_0000000_regex_use/test.cpp
+++ b/tests/std/tests/VSO_0000000_regex_use/test.cpp
@@ -499,7 +499,7 @@ void test_VSO_225160_match_eol_flag() {
 
 void test_VSO_226914_word_boundaries() {
     const test_regex emptyAnchor(&g_regexTester, R"(\b)");
-    emptyAnchor.should_search_match("", "");
+    emptyAnchor.should_search_fail("");
     emptyAnchor.should_search_fail("", match_not_bow);
     emptyAnchor.should_search_fail("", match_not_eow);
     emptyAnchor.should_search_fail("", match_not_bow | match_not_eow);
@@ -1171,6 +1171,12 @@ void test_gh_5253() {
     g_regexTester.should_not_match("a", "()*");
 }
 
+void test_gh_5371() {
+    // GH-5371 <regex>: \b and \B are backwards on empty strings
+    g_regexTester.should_not_match("", "\\b");
+    g_regexTester.should_match("", "\\B");
+}
+
 int main() {
     test_dev10_449367_case_insensitivity_should_work();
     test_dev11_462743_regex_collate_should_not_disable_regex_icase();
@@ -1208,6 +1214,7 @@ int main() {
     test_gh_5192();
     test_gh_5214();
     test_gh_5253();
+    test_gh_5371();
 
     return g_regexTester.result();
 }

From ca633208ce3370681c88192b38c32df898e76fec Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@microsoft.com>
Date: Tue, 1 Apr 2025 14:57:28 -0700
Subject: [PATCH 2/2] Use raw string literals.

---
 tests/std/tests/VSO_0000000_regex_use/test.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/std/tests/VSO_0000000_regex_use/test.cpp b/tests/std/tests/VSO_0000000_regex_use/test.cpp
index 7ef92d2f0fb..2f9d3fec28a 100644
--- a/tests/std/tests/VSO_0000000_regex_use/test.cpp
+++ b/tests/std/tests/VSO_0000000_regex_use/test.cpp
@@ -1173,8 +1173,8 @@ void test_gh_5253() {
 
 void test_gh_5371() {
     // GH-5371 <regex>: \b and \B are backwards on empty strings
-    g_regexTester.should_not_match("", "\\b");
-    g_regexTester.should_match("", "\\B");
+    g_regexTester.should_not_match("", R"(\b)");
+    g_regexTester.should_match("", R"(\B)");
 }
 
 int main() {