it is the first match
but you probably know it by now :-)

The letter after the underscore/hyphen matched in the regex.

There are brackets around the full stop, those brackets are what you can reference with $1 and so on.

It's a shortcut for
value = value || another_value

Just like += is a shortcut for
x = x + y

1. More readable to me and I even think it is even a tad more efficient than negating a boolean creating calling a method;
2. are they working the same in Ruby? Btw, better readability again;
3. More similar to my Python code, I presume;
4. Not sure, too much time ago, but just to be safe if there was no item.

It's Ruby's "conditional assignment operator". It only assigns the value to a variable if it is nil (or false), otherwise it doesn't do anything. It is necessary in this case to preserve the values in @cache every time the fibonacci method is called. If it was a simple assignment operator, @cache would get an empty hash assigned to it when fibonacci is called, and the solution wouldn't work.

No good reason, thanks for the comment

I just think it reads nice there. IFAIK the only difference between || and or is precedence, which doesn't matter there.