[regex] fix uncaught exception when string is like "\\_"

fixes #129062

Author: Zhenhang1213

libcxx/include/regex

@@ -3954,7 +3954,7 @@ _ForwardIterator basic_regex<_CharT, _Traits>::__parse_character_escape(
       ++__first;
       break;
     default:
-      if (*__first != '_' && !__traits_.isctype(*__first, ctype_base::alnum)) {
+      if (!__traits_.isctype(*__first, ctype_base::alnum)) {
         if (__str)
           *__str = *__first;
         else

libcxx/test/std/re/re.alg/re.alg.match/ecma.pass.cpp

@@ -16,6 +16,8 @@
 //                  const basic_regex<charT, traits>& e,
 //                  regex_constants::match_flag_type flags = regex_constants::match_default);
 
+// XFAIL: FROZEN-CXX03-HEADERS-FIXME
+
 #include <regex>
 #include <cassert>
 #include "test_macros.h"
@@ -669,6 +671,21 @@ int main(int, char**)
         assert(m.position(0) == 0);
         assert(m.str(0) == s);
     }
+    {
+        std::cmatch m;
+        const char s[] = "$_se";
+        assert(std::regex_match(s, m, std::regex("\\$\\_se")));
+        assert(m.size() == 1);
+        assert(!m.prefix().matched);
+        assert(m.prefix().first == s);
+        assert(m.prefix().second == m[0].first);
+        assert(!m.suffix().matched);
+        assert(m.suffix().first == m[0].second);
+        assert(m.suffix().second == s + std::char_traits<char>::length(s));
+        assert(m.length(0) >= 0 && static_cast<size_t>(m.length(0)) == std::char_traits<char>::length(s));
+        assert(m.position(0) == 0);
+        assert(m.str(0) == s);
+    }
 
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
     {