I think it's more correct to say that the values always start with "a" followed by an integer. Your regex matches 1 or more digits, found by one or more = signs, followed by a literal double-quote character, etc.
My guess is that we don't want the "+" after the "=" sign; it's spurious in any event and it could be a little misleading.
I think the correct regex would look like this: "a\d+=\"(?<y>.*?)\""
You don't need the backslash in front of the = sign, as it's not a metacharacter, but if you want to do it as a matter of style it won't hurt anything.