用正则表达式匹配空字符串 [英] Matching an empty string with regex

问题描述

我在 stackoverflow 上看到过一些帖子说正则表达式 ^$ 将匹配一个空字符串......所以这让我想......为什么不是这样的:^\s+$ - 这也不行吗?我知道打字更多，但在我看来，它也更有意义.我以前没有使用过很多正则表达式，但随着时间的推移，我对它们的需求似乎越来越大 - 所以我正在接受提示并尝试学习.

解决方案

^\s+$ - 这也不行吗?

不是用于匹配空字符串.一般来说，X+ 表示X 一次或多次.所以，\s+ 不能匹配空字符串——它至少需要一个 \s 才能匹配.

<前>^ \s + $||||字符串开头 ---------------------+ |||空白字符 ------------------+ ||-------------+ 之前的一项或多项 |字符串结尾------------------------------+

现在，X* 的意思是 X 0 次或多次，所以 ^\s*$ 确实会匹配一个空字符串.

<小时>

^\s+$

^\s*$

I've seen posts here on stackoverflow that say that the regex ^$ will match an empty string... So it made me think... why not something like this: ^\s+$ - does that not also work? I know it's more typing, but it in my mind, it also makes more sense. I've not used a whole lot of regex before, but it seems like my need for them is becoming greater as the days go by - so I'm taking the hint and attempting to learn.

解决方案

^\s+$ - does that not also work?

Not for matching an empty string. In general, X+ means X one or more times. So, \s+ cannot match the empty string - it requires at least one \s in order to match.

                                     ^ \s + $
                                     | |  | |
start of string ---------------------+ |  | |
whitespace character ------------------+  | |
one or more of what precedes -------------+ |
end of string ------------------------------+

Now, X* means X 0 or more times, so ^\s*$ would indeed match an empty string.

---

^\s+$

^\s*$

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