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.

I don't quite understand it.

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.

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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.