Underscore-separated digits

[BlueManedHawk]

Some systems use underscores as a digit separator, which this list doesn't have strings for.

[josheinstein]

Was added to C# a few versions ago. The underscores can be placed anywhere, not just in groups of 3. So may be worth testing different groupings to make sure if they're supposed to parse as numbers, they return the same numbers.

[Ferroin]

Erlang and Elixir also support this syntax (Erlang because it was developed in a region where spaces are commonly used for digit grouping, and Elixir because it inherited it from Erlang).

This may also be worth including to catch systems that expect non-numeric input, but fail to catch this particular format and parse it as numeric (though I suspect those cases to be much less common that wanting to accept this format and not doing so correctly).