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Showing content from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/sticky below:

RegExp.prototype.sticky - JavaScript | MDN

Baseline Widely available

The sticky accessor property of RegExp instances returns whether or not the y flag is used with this regular expression.

const str1 = "table football";
const regex1 = new RegExp("foo", "y");

regex1.lastIndex = 6;

console.log(regex1.sticky);
// Expected output: true

console.log(regex1.test(str1));
// Expected output: true

console.log(regex1.test(str1));
// Expected output: false

RegExp.prototype.sticky has the value true if the y flag was used; otherwise, false. The y flag indicates that the regex attempts to match the target string only from the index indicated by the lastIndex property (and unlike a global regex, does not attempt to match from any later indexes).

The set accessor of sticky is undefined. You cannot change this property directly.

For both sticky regexes and global regexes:

• They start matching at lastIndex.
• When the match succeeds, lastIndex is advanced to the end of the match.
• When lastIndex is out of bounds of the currently matched string, lastIndex is reset to 0.

However, for the exec() method, the behavior when matching fails is different:

• When the exec() method is called on a sticky regex, if the regex fails to match at lastIndex, the regex immediately returns null and resets lastIndex to 0.
• When the exec() method is called on a global regex, if the regex fails to match at lastIndex, it tries to match from the next character, and so on until a match is found or the end of the string is reached.

For the exec() method, a regex that's both sticky and global behaves the same as a sticky and non-global regex. Because test() is a simple wrapper around exec(), test() would ignore the global flag and perform sticky matches as well. However, due to many other methods special-casing the behavior of global regexes, the global flag is, in general, orthogonal to the sticky flag.

• String.prototype.matchAll() (which calls RegExp.prototype[Symbol.matchAll]()): y, g and gy are all different.
  • For y regexes: matchAll() throws; [Symbol.matchAll]() yields the exec() result exactly once, without updating the regex's lastIndex.
  • For g or gy regexes: returns an iterator that yields a sequence of exec() results.
• String.prototype.match() (which calls RegExp.prototype[Symbol.match]()): y, g and gy are all different.
  • For y regexes: returns the exec() result and updates the regex's lastIndex.
  • For g or gy regexes: returns an array of all exec() results.
• String.prototype.search() (which calls RegExp.prototype[Symbol.search]()): the g flag is always irrelevant.
  • For y or gy regexes: always returns 0 (if the very beginning of the string matches) or -1 (if the beginning doesn't match), without updating the regex's lastIndex when it exits.
  • For g regexes: returns the index of the first match in the string, or -1 if no match is found.
• String.prototype.split() (which calls RegExp.prototype[Symbol.split]()): y, g, and gy all have the same behavior.
• String.prototype.replace() (which calls RegExp.prototype[Symbol.replace]()): y, g and gy are all different.
  • For y regexes: replaces once at the current lastIndex and updates lastIndex.
  • For g and gy regexes: replaces all occurrences matched by exec().
• String.prototype.replaceAll() (which calls RegExp.prototype[Symbol.replace]()): y, g and gy are all different.
  • For y regexes: replaceAll() throws.
  • For g and gy regexes: replaces all occurrences matched by exec().

const str = "#foo#";
const regex = /foo/y;

regex.lastIndex = 1;
regex.test(str); // true
regex.lastIndex = 5;
regex.test(str); // false (lastIndex is taken into account with sticky flag)
regex.lastIndex; // 0 (reset after match failure)

For several versions, Firefox's SpiderMonkey engine had a bug with regard to the ^ assertion and the sticky flag which allowed expressions starting with the ^ assertion and using the sticky flag to match when they shouldn't. The bug was introduced some time after Firefox 3.6 (which had the sticky flag but not the bug) and fixed in 2015. Perhaps because of the bug, the specification specifically calls out the fact that:

Even when the y flag is used with a pattern, ^ always matches only at the beginning of Input, or (if rer.[[Multiline]] is true) at the beginning of a line.

Examples of correct behavior:

const regex = /^foo/y;
regex.lastIndex = 2;
regex.test("..foo"); // false - index 2 is not the beginning of the string

const regex2 = /^foo/my;
regex2.lastIndex = 2;
regex2.test("..foo"); // false - index 2 is not the beginning of the string or line
regex2.lastIndex = 2;
regex2.test(".\nfoo"); // true - index 2 is the beginning of a line

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