I'll never understand the tempatation to fit your code into as small a space as possible at all costs. Why in the world would you omit white space between symbols and sacrifice performance just to have the most terse answer?

Reraised above with more detail

Please use spoiler flag next time.

Nice but doesn't work if you have two underscores or hyphens in a row.

Awesome !

hard to read

Maybe you need to get more familiar with dealing with arrays, lists and collections functionally ;-)

Also there are ways to make it O(n), so that's a separate problem.

I'll grant that this is a clever use of LINQ to solve the problem. But this is both innefficient - O(n^2) - and hard to read. Using LINQ may make this brief, but it took me over a minute to parse what the code did whereas if it were written more verbosely without LINQ it would likely been quick to understand.

Like another commenter, when I run across code that looks like this, I have to use resharper to translate it to mortal speak. That is a strong smell that the code is not maintainable.

Thanks, I'll fix this

"1\n" should definitely be false, this needs to be tested.

It is failing this test:

Assert.IsFalse("1\n".SignedEightBitNumber());

For some reason $ doesn't work very well with \n having nothing after it. When translating I might have forgotten to remove this case.