[libc++][C++03] Cherry-pick llvm#129348 (llvm#162821)

Author: philnik777

libcxx/include/__cxx03/regex

@@ -3911,7 +3911,7 @@ _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

@@ -7,8 +7,6 @@
 //===----------------------------------------------------------------------===//
 //
 
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
-
 // <regex>
 
 // template <class BidirectionalIterator, class Allocator, class charT, class traits>