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.

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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:

const 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 Language Specification
# sec-properties-of-regexp-instances

Browser compatibility

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See also