Best of Modern JavaScript — Regex y Flag

The y flag acts differently in different methods.

Since 2015, JavaScript has improved immensely.

It’s much more pleasant to use it now than ever.

In this article, we’ll look at new JavaScript regex features.

Search and the y Flag

The String.prototype.search method ignores the y flag.

lastIndex isn’t changed by it.

For example, if we have:

const REGEX = /foo/y;

REGEX.lastIndex = 8;
const match = 'barfoobarfoo'.search(REGEX);
console.log(match.index);
console.log(REGEX.lastIndex);

Then match us undefined and lastIndex stays at 8.

We can only search from the beginning of the string with it.

The String.prototype.match works differently depending on whether the g flag is added with y .

If g isn’t, it works like exec .

If g is set, then it returns an array with string parts that’s matched or null .

For instance, if we write:

const REGEX = /a/;

REGEX.lastIndex = 7;
console.log('abab'.match(REGEX).index); 
console.log(REGEX.lastIndex);

Then index is 0, and lastIndex is 7.

It gets the first if no flag is added.

If we add the y flag:

const REGEX = /a/y;

REGEX.lastIndex = 2;
console.log('abab'.match(REGEX).index); 
console.log(REGEX.lastIndex);

Then the match is only returned if the search starts from the lastIndex .

The index must be exact.

lastIndex will then be updated after the match is found.

If the g flag is set, then match will match all the substrings and put them in the array.

For example, we can write:

const REGEX = /a|b/g;

REGEX.lastIndex = 0;
console.log('abab'.match(REGEX)); 
console.log(REGEX.lastIndex);

And match returns [“a”, “b”, “a”, “b”] .

lastIndex is still 0.

lastIndex is ignored with the g flag enabled.

The result is the same with the gy flags added.

match and lastIndex all give us the same result.

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