JavaScript RegExps中"y"粘性模式修饰符的作用是什么? [英] What is the purpose of the 'y' sticky pattern modifier in JavaScript RegExps?

问题描述

MDN为JavaScript RegExp引入了"y"粘滞标记.这是文档摘录:

MDN introduced the 'y' sticky flag for JavaScript RegExp. Here is a documentation excerpt:

y

粘性；仅与目标字符串中此正则表达式的lastIndex属性指示的索引匹配(并且不尝试与任何后续索引匹配).

sticky; matches only from the index indicated by the lastIndex property of this regular expression in the target string (and does not attempt to match from any later indexes).

还有一个例子:

var text = 'First line\nSecond line';
var regex = /(\S+) line\n?/y;

var match = regex.exec(text);
console.log(match[1]);        // prints 'First'
console.log(regex.lastIndex); // prints '11'

var match2 = regex.exec(text);
console.log(match2[1]);       // prints 'Second'
console.log(regex.lastIndex); // prints '22'

var match3 = regex.exec(text);
console.log(match3 === null); // prints 'true'

但是在这种情况下， g 全局标志的使用实际上没有任何区别:

But there isn't actually any difference between the usage of the g global flag in this case:

var text = 'First line\nSecond line';
var regex = /(\S+) line\n?/g;

var match = regex.exec(text);
console.log(match[1]);        // prints 'First'
console.log(regex.lastIndex); // prints '11'

var match2 = regex.exec(text);
console.log(match2[1]);       // prints 'Second'
console.log(regex.lastIndex); // prints '22'

var match3 = regex.exec(text);
console.log(match3 === null); // prints 'true'

相同的输出.因此，我猜想可能还有其他关于'y'标志的问题，并且MDN的示例似乎不是此修饰符的真实用例，因为它似乎可以代替'g'全局修饰符.

Same output. So I guess there might be something else regarding the 'y' flag and it seems that MDN's example isn't a real use-case for this modifier, as it seems to just work as a replacement for the 'g' global modifier here.

那么，这个实验性"y"粘滞标记的实际用例是什么?仅从RegExp.lastIndex属性进行匹配"的目的是什么?和 RegExp.prototype.exec 一起使用时，它与'g'有何不同?

So, what could be a real use-case for this experimental 'y' sticky flag? What's its purpose in "matching only from the RegExp.lastIndex property" and what makes it differ from 'g' when used with RegExp.prototype.exec?

感谢您的关注.

推荐答案

中描述了 y 和 g 之间的差异="https://github.com/mjavascript/practical-modern-javascript/blob/master/ch07.asciidoc#sticky-matching-flag-y" rel ="nofollow noreferrer">实用的现代JavaScript :

The difference between y and g is described in Practical Modern JavaScript:

粘滞标记像 g 一样前进 lastIndex ，但是仅当找到匹配项时从 lastIndex 开始，没有前向搜索.添加了粘滞标记以提高使用以下命令编写词法分析器的性能JavaScript ...

The sticky flag advances lastIndex like g but only if a match is found starting at lastIndex, there is no forward search. The sticky flag was added to improve the performance of writing lexical analyzers using JavaScript...

对于实际用例

可以用于要求从位置 n 开始的正则表达式匹配，其中 n 是 lastIndex 设置的.如果是非多行常规表达式，带有粘滞标记的 lastIndex 值为 0 效果与以 ^ 开始正则表达式相同要求匹配从搜索到的文本的开头开始.

It could be used to require a regular expression match starting at position n where n is what lastIndex is set to. In the case of a non-multiline regular expression, a lastIndex value of 0 with the sticky flag would be in effect the same as starting the regular expression with ^ which requires the match to start at the beginning of the text searched.

这是该博客中的一个示例，其中在 test 方法调用之前操作了 lastIndex 属性，从而强制使用不同的匹配结果:

And here is an example from that blog, where the lastIndex property is manipulated before the test method invocation, thus forcing different match results:

var searchStrings, stickyRegexp;

stickyRegexp = /foo/y;

searchStrings = [
    "foo",
    " foo",
    "  foo",
];
searchStrings.forEach(function(text, index) {
    stickyRegexp.lastIndex = 1;
    console.log("found a match at", index, ":", stickyRegexp.test(text));
});

结果:

"found a match at" 0 ":" false
"found a match at" 1 ":" true
"found a match at" 2 ":" false

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