为什么Javascript的regex.exec（）并不总是返回相同的值？ [英] Why does Javascript's regex.exec() not always return the same value?

问题描述

在Chrome或Firebug控制台中：

In the Chrome or Firebug console:

reg = /ab/g
str = "abc"
reg.exec(str)
   ==> ["ab"]
reg.exec(str)
   ==> null
reg.exec(str)
   ==> ["ab"]
reg.exec(str)
   ==> null

exec在某种程度上是有状态的，取决于它上一次返回的内容吗？或者这只是一个错误？我无法让它一直发生。例如，如果上面的'str'是abc abc则不会发生。

Is exec somehow stateful and depends on what it returned the previous time? Or is this just a bug? I can't get it to happen all the time. For example, if 'str' above were "abc abc" it doesn't happen.

推荐答案

一个JavaScript RegExp 对象是有状态的。

A JavaScript RegExp object is stateful.

当正则表达式是全局的时，如果你调用一个方法相同的正则表达式对象，它将从索引超过最后一场比赛结束。

When the regex is global, if you call a method the same regex object, it will start from the index past the end of the last match.

当找不到更多匹配项时，索引将重置为 0 自动。

When no more matches are found, the index is reset to 0 automatically.

要手动重置，请设置 lastIndex 属性。

To reset it manually, set the lastIndex property.

reg.lastIndex = 0;

---

这可能是一个非常有用的功能。如果需要，您可以在字符串中的任何位置开始评估，或者如果在循环中，您可以在所需数量的匹配后停止评估。

---

This can be a very useful feature. You can start the evaluation at any point in the string if desired, or if in a loop, you can stop it after a desired number of matches.

这是一个在循环中使用正则表达式的典型方法的演示。它通过执行赋值作为循环条件，当没有更多匹配时， exec 返回 null 这一事实。

Here's a demonstration of a typical approach to using the regex in a loop. It takes advantage of the fact that exec returns null when there are no more matches by performing the assignment as the loop condition.

var re = /foo_(\d+)/g,
    str = "text foo_123 more text foo_456 foo_789 end text",
    match,
    results = [];

while (match = re.exec(str))
    results.push(+match[1]);

DEMO： http://jsfiddle.net/pPW8Y/

如果您不喜欢分配的位置，可以重新设置循环，例如......

If you don't like the placement of the assignment, the loop can be reworked, like this for example...

var re = /foo_(\d+)/g,
    str = "text foo_123 more text foo_456 foo_789 end text",
    match,
    results = [];

do {
    match = re.exec(str);
    if (match)
        results.push(+match[1]);
} while (match);

DEMO： http://jsfiddle.net/pPW8Y/1/

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