正则表达式'标志'是什么？ [英] What does regex' flag 'y' do?

问题描述

MDN说：

要执行粘性搜索，
匹配从当前
开始在目标字符串中的位置，使用
y flag。

To perform a "sticky" search, that matches starting at the current position in the target string, use the y flag.

我不太明白。

推荐答案

正则表达式对象具有 lastIndex 属性，该属性以不同方式使用，具体取决于 g （全局）和 y （粘性）标志。 y （粘性）标志告诉正则表达式在 lastIndex 和仅 at lastIndex （不在字符串的前面或后面）。

Regular expression objects have a lastIndex property, which is used in different ways depending on the g (global) and y (sticky) flags. The y (sticky) flag tells the regular expression to look for a match at lastIndex and only at lastIndex (not earlier or later in the string).

示例值1024字：

var str =  "a0bc1";
// Indexes: 01234

var rexWithout = /\d/;
var rexWith    = /\d/y;

// Without:
rexWithout.lastIndex = 2;          // (This is a no-op, because the regex
                                   // doesn't have either g or y.)
console.log(rexWithout.exec(str)); // ["0"], found at index 1, because without
                                   // the g or y flag, the search is always from
                                   // index 0

// With, unsuccessful:
rexWith.lastIndex = 2;             // Says to *only* match at index 2.
console.log(rexWith.exec(str));    // => null, there's no match at index 2,
                                   // only earlier (index 1) or later (index 4)

// With, successful:
rexWith.lastIndex = 1;             // Says to *only* match at index 1.
console.log(rexWith.exec(str));    // => ["0"], there was a match at index 1.

// With, successful again:
rexWith.lastIndex = 4;             // Says to *only* match at index 4.
console.log(rexWith.exec(str));    // => ["1"], there was a match at index 4.

.as-console-wrapper {
  max-height: 100% !important;
}

兼容性注释：

Firefox的SpiderMonkey JavaScript引擎已经拥有 y 多年来的标志，但在ES2015（2015年6月）之前它不是规范的一部分。此外，很长一段时间Firefox已经处理的错误y 标志关于 ^ 断言，但它在Firefox 43（有bug）和Firefox 47之间修复（没有）。很老版本的Firefox（比如3.6）确实有 y 并且没有错误，所以这是后来发生的回归（未定义<$的行为） c $ c> y flag），然后得到修复。

Firefox's SpiderMonkey JavaScript engine has had the y flag for years, but it wasn't part of the specification until ES2015 (June 2015). Also, for quite a while Firefox had a bug in the handling of the y flag with regard to the ^ assertion, but it was fixed somewhere between Firefox 43 (has the bug) and Firefox 47 (doesn't). Very old versions of Firefox (say, 3.6) did have y and didn't have the bug, so it was a regression that happened later (not defined behavior for the y flag), and then got fixed.

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