Eloquent Javascript循环使用RegExp匹配

Question

The following example is a bit confusing to me:

var text = "A string with 3 numbers in it ... 42 and 88.";
var number = /\b(\d+)\b/g;
var match;
while (match = number.exec(text)){
    console.log("Found", match[1], "at", match.index);
}

Specifically, I don't understand how this has a "looping" effect. How does it run through all the matches within one string if it keeps calling match[1] . Is there some kind of side effect with exec that I am unaware of?

Edit: I still would like an answer to how match[1] is working. How does match[1] produce any answer? When I test this type of thing myself, I get undefined , look

> var y = /\d+/g.exec('5')
undefined
> y
[ '5', index: 0, input: '5' ]
> y[1]
undefined

Whats going on here? Wouldn't it be y[0], or in the case above, match[0]? Like:

> y[0]
'5'

Answer 1

The RegExp object remembers the last matched position with lastIndex property.

Quoting MDN Documentation ,

If your regular expression uses the "g" flag, you can use the exec() method multiple times to find successive matches in the same string. When you do so, the search starts at the substring of str specified by the regular expression's lastIndex property ( test() will also advance the lastIndex property).

Important Note: The first part of the first line of the quoted section is important. If your regular expression uses the "g" flag . Only if the RegEx has g flag you will get this behavior.