RegExp对象保留先前的匹配项

Question

Consider this Javascript regular expression code:

v = 'The dog jumped over the moon';

// Matches
if (v.match(/(jump)ed/)) {
    alert('Initial: ' + RegExp.$1); // Alerts "jump"
}

// Obviously not matched, but RegExp object retains previous match
if (v.match(/(gygyujgujy)/)) {
    alert(RegExp.$1);
}

// Alerts "jump" but shouldn't this be null/false etc?
alert('Final: ' + RegExp.$1);

The second regular expression, despite not matching, holds on to the first regular expressions matches.

Shouldn't the RegExp object be empty because of the no match on the second regular expression?

Answer 1

from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/n

"The $1, ..., $9 properties are static, they are not a property of an individual regular expression object. Instead, you always use them as RegExp.$1, ..., RegExp.$9.

The values of these properties are read-only and modified whenever successful matches are made."

Since your 2nd if-statement never matches anything, no new value will be assigned to $1

Answer 2

They are non-standard properties and documented as:

The values of these properties are read-only and modified whenever successful matches are made.

There is no match for gygyujgujy so the existing value remains.