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

@mattbrendzel, @nbeck the problem lies with the parentheses on the class name Sudoku() and passing self as parameter to the method. Try removing these. Also, write an initialize method.

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.

Hey Giacomo, a few questions:

1. why use item[0].length>0 instead of !item[0].empty?
2. why use "and" and "or", instead of && and || on the same line?
3. why "0123456789".index(item[0][item[0].length-1])!=nil instead of item[0][item[0].length-1] =~ /\d/ != nil
4. Why is it necessary to say (res[item[1]] or 0)?

Good kata, just a few spelling and grammatical issues. The title should be "Simplifying multilinear polynomials".
There are also a few grammatical issues in the description that I can point out, if you'd like.

Thanks, I'll do that

Thanks, I'll do that

@GiacomoSorbi haven't really contributed to this site before, but is there any way to suggest spelling and grammatical improvements to the author? There are a few errors, including in the title

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.

From the description, your function should be case insensitive to the vowels.

What is $1?

What is $1?