What does regex' flag 'y' do?

Question

MDN says:

To perform a \"sticky\" search, that matches starting at the current position in the target string, use the y flag.

Answer 1

Regular expression objects have a lastIndex property, which is used in different ways depending on the g (global) and y (sticky) flags. The y (sticky) flag tells the regular expression to look for a match at lastIndex and only at lastIndex (not earlier or later in the string).

Examples are worth 1024 words:

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;
}

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Compatibility Note:

Firefox's SpiderMonkey JavaScript engine has had the y flag for years, but it wasn't part of the specification until ES2015 (June 2015). Also, for quite a while Firefox had a bug in the handling of the y flag with regard to the ^ assertion, but it was fixed somewhere between Firefox 43 (has the bug) and Firefox 47 (doesn't). Very old versions of Firefox (say, 3.6) did have y and didn't have the bug, so it was a regression that happened later (not defined behavior for the y flag), and then got fixed.