The C++ Standard reserves names beginning with an underscore followed by a capital letter in all scopes.
Does this apply to user literal operators?
e.g.
int _MyInt; // reserved, violation
template < char... >
auto operator "" _MyInt ( ); // reserved???
No, it allows the use of an underscore followed by a capital letter (which is otherwise a reserved identifier).
I've only found an example, not a formal paragraph, in the standard supporting the above:
double operator""_Bq(long double); // OK: does not use the reserved identifier _Bq
double operator"" _Bq(long double); // uses the reserved identifier _Bq
So, as long as you don't put a space between "" and _Ud it's ok - according to the example that is.