Enforce the use of u or v flag on RegExp

๐Ÿ’ก hasSuggestions

Some problems reported by this rule are manually fixable by editor suggestions

RegExp u flag has two effects:

  1. Make the regular expression handling UTF-16 surrogate pairs correctly.

    Especially, character range syntax gets the correct behavior.

    /^[๐Ÿ‘]$/.test("๐Ÿ‘") //โ†’ false
    /^[๐Ÿ‘]$/u.test("๐Ÿ‘") //โ†’ true
  2. Make the regular expression throwing syntax errors early as disabling Annex B extensions.

    Because of historical reason, JavaScript regular expressions are tolerant of syntax errors. For example, /\w{1, 2/ is a syntax error, but JavaScript doesnโ€™t throw the error. It matches strings such as "a{1, 2" instead. Such a recovering logic is defined in Annex B.

    The u flag disables the recovering logic Annex B defined. As a result, you can find errors early. This is similar to the strict mode.

The RegExp v flag, introduced in ECMAScript 2024, is a superset of the u flag, and offers two more features:

  1. Unicode properties of strings

    With the Unicode property escape, you can use properties of strings.

    const re = /^\p{RGI_Emoji}$/v;
    // Match an emoji that consists of just 1 code point:
    re.test('โšฝ'); // '\u26BD'
    // โ†’ true โœ…
    // Match an emoji that consists of multiple code points:
    re.test('๐Ÿ‘จ๐Ÿพโ€โš•๏ธ'); // '\u{1F468}\u{1F3FE}\u200D\u2695\uFE0F'
    // โ†’ true โœ…
  2. Set notation

    It allows for set operations between character classes.

    const re = /[\p{White_Space}&&\p{ASCII}]/v;
    re.test('\n'); // โ†’ true
    re.test('\u2028'); // โ†’ false

Therefore, the u and v flags let us work better with regular expressions.

Rule Details

This rule aims to enforce the use of u or v flag on regular expressions.

Examples of incorrect code for this rule:

Open in Playground
/*eslint require-unicode-regexp: error */

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

Examples of correct code for this rule:

Open in Playground
/*eslint require-unicode-regexp: error */

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

const e = /aaa/v
const f = /bbb/giv
const g = new RegExp("ccc", "v")
const h = new RegExp("ddd", "giv")

// This rule ignores RegExp calls if the flags could not be evaluated to a static value.
function i(flags) {
    return new RegExp("eee", flags)

When Not To Use It

If you donโ€™t want to warn on regular expressions without either a u or a v flag, then itโ€™s safe to disable this rule.


This rule was introduced in ESLint v5.3.0.

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