js正则表达式量词和全局标志输出空字符串作为匹配数组中的最后一个元素

Question

I have:

var str = 'ddd';
var r = /d*/g;
console.log(str.match(r))

the output in my console is an array of two; first is a string containing 'ddd' and second its a empty string. I know what * means (0 or more) but why it outputs that empty string? I would expect to ouptut just one string 'ddd' . Once it matches for d key, why it just continue with that empty string?

If I call without the g flag, it outputs what I really expected to do with it.

I also know that g means, global search, it iterates through all elements of str , but how that, when my search was completed by the first match?

Answer 1

The pattern d* means "zero or more occurrences of 'd'". Zero occurrences of 'd' is the empty string. The greedy * first matches all the 'd' characters in the string, and then matches the nothing between the last 'd' and the end of the string.

The g flag on the pattern tells the regular expression matching mechanism to iteratively re-try the match. The first iteration matches all of the 'd' characters. At that point, the remaining string is basically the empty string; a string with zero length. The match succeeds in the same way that "".match(/d*/g) would succeed.

Answer 2

It's logical that you got two matches here:

• First one is 'ddd' :

  which is logical with d* .

• Second one is ' ' :

  which is logical too with d* , because the * means zero or many times and that's zero d here, and it's not a white space, just an empty string so basically any string will be matching it.

You should have been using d+ instead.

EDIT:

You can see it in this DEMO that with ddd the matches are:

1. [0-3] ddd

2. [3-3] ``

It's taking the last of the string as a match here with the index : [3-3]