正则表达式星号用法 [英] Regex asterisk usage

问题描述

根据 MDNx* : *匹配前一项 x 0 次或更多次.*

本质上前面的字符应该是完全可选的.无论是否存在，字符串都会被匹配.那么为什么会这样:

Essentially the preceding characters should be completely optional. The string will be matched whether they exist or not. So why is it that:

1.

var text  = "foobar";
var re    = /q*/;
var found = text.match(re);
console.log(found); // prints '["", index: 0, input: "foobar"]'

但是

var re    = /qz*/;
console.log(found); // prints 'null'

两个表达式同样不存在，因此应该匹配 0 次并且应该返回 '""'.

Both expressions equally don't exist and thus should be matched 0 times and '""' should be returned.

或者:

2.

var re    = /fz*/;
console.log(found); // prints '["f", index: 0, input: "foobar"]'

但是

var re    = /fzq*/;
console.log(found); // prints 'null'

这里发生了什么?根据我的理解 'fz​​q' 不存在所以应该匹配 0 次，并且应该返回 '""' ，对吧?如果它以某种方式匹配每个字符而不是整个字符串，'fz​​q*' 应该返回与 'fz​​' - 'f'<相同的结果/code> 匹配一次，其余匹配 0 次.但显然这不是正在发生的事情.

What's happening here? From my understanding 'fzq' doesn't exist so should be matched 0 times, and '""' should be returned, right? And if it somehow matches on per character basis instead of the whole string, 'fzq*' should return the same result as 'fz' - 'f' is matched once and the rest is matched 0 times. But apparently this is not what's happening.

有人能解释一下这里到底发生了什么吗?

Could someone shed some light on what the hell is happening here?

推荐答案

x* : 匹配前一项 x 0 次或更多次.

在 * 被匹配零次或多次之前，你对 everything 误解了这个语句.

You misunderstood this statement with everything before * is matched zero or more times.

当您匹配 q* 时，正则表达式将在输入字符串中搜索 q.由于 foobar 中没有 q，它将返回一个空字符串.

When you match q*, the regex will search for q in the input string. As there is no q in the foobar, it'll return an empty string.

当您尝试匹配 qz* 时，正则表达式将尝试在 q<之后搜索任意数量的 zs/强>.由于输入字符串中没有 q，它将返回 null.

When you try to match qz*, the regex will try to search for any number of zs after q. As there is no q in the input string, it'll return null.

要通过 * 或任何其他量词匹配更多字符，您需要使用括号将它们分组.

To match more characters by * or any other quantifiers, you need to group them using parenthesis.

(qz)*

这意味着，匹配字符串 qz 零次或多次.

which means, match the string qz zero or more times.

要了解正则表达式，您可以使用 https://regex101.com/ 站点.

To understand a regex, you can use https://regex101.com/ site.

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