W3cubDocs

/JavaScript

RegExp: lastIndex

lastIndex is a read/write integer property of RegExp instances that specifies the index at which to start the next match.

Note that lastIndex is not a property of the RegExp prototype but is instead only exposed from RegExp instances.

Property attributes of RegExp: lastIndex
Writable yes
Enumerable no
Configurable no

Description

This property is set only if the regular expression instance used the g flag to indicate a global search, or the y flag to indicate a sticky search. The following rules apply when test() and exec() are called on a given input:

  • If lastIndex is greater than the length of the input, exec() or test() will not find a match, and lastIndex will be set to 0.
  • If lastIndex is equal to or less than the length of the input, exec() or test() will attempt to match the input starting from lastIndex.
    • If exec() or test() find a match, then lastIndex will be set to the position of the end of the matched string in the input.
    • If exec() or test() do not find a match, then lastIndex will be set to 0.

Examples

Using lastIndex

Consider the following sequence of statements:

var re = /(hi)?/g;

Matches the empty string.

console.log(re.exec('hi'));
console.log(re.lastIndex);

Returns ["hi", "hi"] with lastIndex equal to 2.

console.log(re.exec('hi'));
console.log(re.lastIndex);

Returns ["", undefined], an empty array whose zeroth element is the match string. In this case, the empty string because lastIndex was 2 (and still is 2) and hi has length 2.

Specifications

Browser compatibility

Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet Deno Node.js
lastIndex
1
12
1
5.5
5
1
1
18
4
10.1
1
1.0
1.0
0.10.0

See also

© 2005–2021 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/lastIndex