it would conflict with user defined literals which were there before: auto operator""_01(unsigned long long l) { return 0; } int x = 100_01; // x ==

it would conflict with user defined literals which were there before: auto operator""_01(unsigned long long l) { return 0; } int x = 100_01; // x == 0 so, as always, backward compat :D (grepping in my ~ I see at least one occurence of `operator "" _0b` and if you allow `_0b` it's going to be hard to justify not allowing `_0123456`).