regexp.lastIndex

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

Property attributes of regexp.lastIndex
Writable yes
Enumerable no
Configurable no

Syntax

JavaScript
<var>regExpObj</var>.lastIndex

Description

This property is set only if the regular expression instance used the "g" flag to indicate a global search. The following rules apply:

  • If lastIndex is greater than the length of the string, test() and exec() fail, then lastIndex is set to 0.
  • If lastIndex is equal to the length of the string and if the regular expression matches the empty string, then the regular expression matches input starting at lastIndex.
  • If lastIndex is equal to the length of the string and if the regular expression does not match the empty string, then the regular expression mismatches input, and lastIndex is reset to 0.
  • Otherwise, lastIndex is set to the next position following the most recent match.

Examples

Using lastIndex

Consider the following sequence of statements:

JavaScript
var re = /(hi)?/g;

Matches the empty string.

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

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

JavaScript
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

Specification Status Comment
ECMAScript 3rd Edition (ECMA-262) Standard Initial definition. Implemented in JavaScript 1.2. JavaScript 1.5: lastIndex is a property of a RegExp instance, not the RegExp object.
ECMAScript 5.1 (ECMA-262)
The definition of 'RegExp.lastIndex' in that specification.
Standard  
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'RegExp.lastIndex' in that specification.
Standard  
ECMAScript 2017 Draft (ECMA-262)
The definition of 'RegExp.lastIndex' in that specification.
Draft  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

See also

License

© 2016 Mozilla 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

JavaScript Property Reference RegExp Regular Expressions