正则表达式的区别：（\\ w +）？ 和（\\ w *）

Question

Is there any difference between (\\w+)? and (\\w*) in regex?

It seems the same, doesn't it?

Answer 1

(\\w+)? and (\\w*) both match the same (0..+inf word characters)

However, there is a slight difference:

In the first case, if this part of the regex matches "" , the capturing group is absent. In the second case, it is empty. In some languages, the former manifests as a null while the latter should always be "" .

In Javascript, for example,

/(\w*)/.exec("")  // ["", ""]
/(\w+)?/.exec("") // ["", undefined]

In PHP ( preg_match ), in the former case, the corresponding key is simply absent in the matches array: http://3v4l.org/DB6p3#v430