<regex>: Revise caret parsing in basic and grep mode

stl/inc/regex

@@ -3874,10 +3874,9 @@ void _Parser<_FwdIt, _Elem, _RxTraits>::_Trans() { // map character to meta-char
         break;
 
     case _Meta_caret:
-        if ((_L_flags & _L_anch_rstr) && !_Nfa._Beg_expr()) {
-            _Mchar = _Meta_chr;
-        }
-
+        // A caret can always negate a bracket expression,
+        // but _L_anch_rstr (used by basic/grep) restricts caret anchors to the beginning.
+        // We'll handle that restriction when we're about to add a bol node.
         break;
 
     case _Meta_dlr:
@@ -4435,7 +4434,7 @@ bool _Parser<_FwdIt, _Elem, _RxTraits>::_Alternative() { // check for valid alte
             _Next();
             _Quant = _Wrapped_disjunction();
             _Expect(_Meta_rpar, regex_constants::error_paren);
-        } else if (_Mchar == _Meta_caret) { // add bol node
+        } else if (_Mchar == _Meta_caret && (!(_L_flags & _L_anch_rstr) || _Nfa._Beg_expr())) { // add bol node
             _Nfa._Add_bol();
             _Next();
             _Quant = false;

tests/std/tests/VSO_0000000_regex_use/test.cpp

@@ -669,6 +669,43 @@ void test_gh_5160() {
     neg_regex.should_search_fail(L"xxxYxx\x2009xxxZxxx"); // U+2009 THIN SPACE
 }
 
+void test_gh_5165_syntax_option(const syntax_option_type basic_or_grep) {
+    g_regexTester.should_not_match("yx", "y[^x]", basic_or_grep);
+    g_regexTester.should_match("yz", "y[^x]", basic_or_grep);
+    g_regexTester.should_match("y^", "y[^x]", basic_or_grep);
+
+    g_regexTester.should_match("yx", "y[x^]", basic_or_grep);
+    g_regexTester.should_not_match("yz", "y[x^]", basic_or_grep);
+    g_regexTester.should_match("y^", "y[x^]", basic_or_grep);
+
+    g_regexTester.should_not_match("yx", "y[^x^]", basic_or_grep);
+    g_regexTester.should_match("yz", "y[^x^]", basic_or_grep);
+    g_regexTester.should_not_match("y^", "y[^x^]", basic_or_grep);
+
+    {
+        const test_regex no_anchor(&g_regexTester, "meo[wW]", basic_or_grep);
+        no_anchor.should_search_match("meow_machine", "meow");
+        no_anchor.should_search_match("homeowner", "meow");
+    }
+    {
+        const test_regex beginning_anchor(&g_regexTester, "^meo[wW]", basic_or_grep);
+        beginning_anchor.should_search_match("meow_machine", "meow");
+        beginning_anchor.should_search_fail("homeowner");
+    }
+    {
+        const test_regex middle_anchor(&g_regexTester, "me^o[wW]", basic_or_grep);
+        middle_anchor.should_search_fail("meow_machine");
+        middle_anchor.should_search_fail("homeowner");
+        middle_anchor.should_search_match("home^owner", "me^ow");
+    }
+}
+
+void test_gh_5165() {
+    // GH-5165: Caret ^ should negate character classes in basic regular expressions
+    test_gh_5165_syntax_option(basic);
+    test_gh_5165_syntax_option(grep);
+}
+
 int main() {
     test_dev10_449367_case_insensitivity_should_work();
     test_dev11_462743_regex_collate_should_not_disable_regex_icase();
@@ -699,6 +736,7 @@ int main() {
     test_gh_4995();
     test_gh_5058();
     test_gh_5160();
+    test_gh_5165();
 
     return g_regexTester.result();
 }