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.

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.

• RegExp.prototype.dotAll
• RegExp.prototype.global
• RegExp.prototype.hasIndices
• RegExp.prototype.ignoreCase
• RegExp.prototype.multiline
• RegExp.prototype.source
• RegExp.prototype.sticky
• RegExp.prototype.unicode