Что делает флаг регулярного выражения «y»?

var str =  "a0bc1";
// Indexes: 01234

var rexWithout = /\d/;
var rexWith    = /\d/y;

// Without:
rexWithout.lastIndex = 2;          // (This is a no-op, because the regex
                                   // doesn't have either g or y.)
console.log(rexWithout.exec(str)); // ["0"], found at index 1, because without
                                   // the g or y flag, the search is always from
                                   // index 0

// With, unsuccessful:
rexWith.lastIndex = 2;             // Says to *only* match at index 2.
console.log(rexWith.exec(str));    // => null, there's no match at index 2,
                                   // only earlier (index 1) or later (index 4)

// With, successful:
rexWith.lastIndex = 1;             // Says to *only* match at index 1.
console.log(rexWith.exec(str));    // => ["0"], there was a match at index 1.

// With, successful again:
rexWith.lastIndex = 4;             // Says to *only* match at index 4.
console.log(rexWith.exec(str));    // => ["1"], there was a match at index 4.

.as-console-wrapper {
  max-height: 100% !important;
}