It is recommended to use the u flag with regular expressions.

The u flag has two effects: - It enables correct handling of UTF-16 surrogate pairs. - It ensures the correct behavior of regex character ranges.

/^[👍]$/.test("👍") //→ false
/^[👍]$/u.test("👍") //→ true

For historical reasons, JavaScript regular expressions tolerate syntax errors. For example, /\w{1, 2/ is a regex that would throw a syntax error, but JavaScript chooses not to. It matches strings such as "a{1, 2" instead. This behaviour is defined in Annex B of the Javascript specification.

The u flag disables the recovering logic Annex B of the Javascript specification. This way, you can find errors early. It can therefore be thought of as a "strict mode" for RegEx literals.

This issue is raised when: - A regular expression contains unicode property escapes i.e \p{<property-name>} - A regular expression contains 4 bytes characters like emojis or some special characters

Bad Practice

const a = /aaa/
const b = /bbb/gi
const c = new RegExp("ccc")
const d = new RegExp("ddd", "gi")

Recommended

const a = /aaa/u
const b = /bbb/giu
const c = new RegExp("ccc", "u")
const d = new RegExp("ddd", "giu")

// This rule ignores RegExp calls if the flags are not a compile time constant.
function f(flags) {
    return new RegExp("eee", flags)
}

References

willtheorangeguy / Note-Style-Guide

No ignore rules found