〜限り
as long as; as far as; unless (ない限り)
Plain form + 限り · ない-form + 限り
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限り means "limit", and after a plain form it sets the boundary of a condition.
As long as: 生きている限り、あなたのことは忘れない, "as long as I live, I won't forget you". The second part holds true while the first does.
As far as: 私が知っている限り、彼はまじめな人だ, "as far as I know, he's a serious person". Common with 知る, 見る and 聞く.
Unless, with the ない-form: 雨が降らない限り、試合は行われる, "unless it rains, the match will go ahead".
The set phrase できる限り means "as much as possible", like できるだけ.
Examples
生きている限り、あなたのことは忘れない。
As long as I live, I won't forget you.
私が知っている限り、彼はまじめな人だ。
As far as I know, he's a serious person.
雨が降らない限り、試合は行われる。
Unless it rains, the match will go ahead.
できる限り、お手伝いします。
I'll help as much as I can.
練習しない限り、上手にはならない。
Unless you practise, you won't get better.