Good news (IDE 1.8.4): Sketch names starting with a digit are now allowed
I appreciate that new feature very much, and immediately I could make use of it.
Well, the full truth is
The IDE now accepts including external files whose file names are beginning with a digit.
The lesser good news is: you cannot create them from within the IDE.
Whenever you press "New Tab" (Ctrl-Shift-N) the IDE does not accept any digit char as a first char.
So you have to give it any arbitrary name and use the operating system to rename it to one starting with a digit.
And close and reopen your sketch, of course.
If I had one more wish to ask for:
suppose you have the tabs 8.h, 9.h, 10.h, 11.h
they will be shown in this order: 10.h, 11.h, 8.h, 9.h
Is it too much to ask for showing the tabs in numerical order rather than alphabetical order.
Have a look at the operating system: the explorer will show files names in correct (numerical) order.
I have reported the issue of not being able to enter a number as the first character of a tab name here:
I suppose since numbered tabs are not currently officially supported it would be best to wait to see what happens with that issue before requesting the ordering improvement. In the meantime you can add a leading zero.
Thanks for reporting this Klausj!
The issue of the tab name dialog not allowing a number as the first character has now been solved. It is already available in the hourly build and will also be in the future Arduino IDE releases.