[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.regex/re.regex.construct/ptr.pass.cpp

@@ -25,12 +25,21 @@ test(const CharT* p, unsigned mc)
     assert(r.mark_count() == mc);
 }
 
+template <class CharT>
+void
+test1(const CharT* p, const CharT * testString)
+{
+  std::regex regexObj(p);
+  assert(std::regex_match(testString, regexObj));
+}
+
 int main(int, char**)
 {
     test("\\(a\\)", 0);
     test("\\(a[bc]\\)", 0);
     test("\\(a\\([bc]\\)\\)", 0);
     test("(a([bc]))", 2);
+    test1("\\$\\_se", "$_se");
 
-  return 0;
+    return 0;
 }