[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

@@ -1305,6 +1305,12 @@ 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);
+    }
 #endif // TEST_HAS_NO_WIDE_CHARACTERS
 
   return 0;