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:

  • 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 or less than the length of the string and if the regular expression matches the empty string, then the regular expression matches input starting from 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:

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

Specification
ECMAScript (ECMA-262)
The definition of 'lastIndex' in that specification.

Browser compatibility

BCD tables only load in the browser

See also