require-unicode-regexp

Enforce the use of u or v flag on regular expressions

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.

   /^[👍]$/.test("👍") //→ false
   /^[👍]$/u.test("👍") //→ true
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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 doesn’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('⚽'); // '\u26BD'
   // → true ✅
   
   // Match an emoji that consists of multiple code points:
   re.test('👨🏾‍⚕️'); // '\u{1F468}\u{1F3FE}\u200D\u2695\uFE0F'
   // → true ✅
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2. Set notation

   It allows for set operations between character classes.

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

/*eslint require-unicode-regexp: error */

const a = /aaa/
const b = /bbb/gi
const c = new RegExp("ccc")
const d = new RegExp("ddd", "gi")
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Examples of correct code for this rule:

/*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)
}
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Options

This rule has one object option:

• "requireFlag": "u"|"v" requires a particular Unicode regex flag

requireFlag: “u”

The u flag may be preferred in environments that do not support the v flag.

Examples of incorrect code for this rule with the { "requireFlag": "u" } option:

/*eslint require-unicode-regexp: ["error", { "requireFlag": "u" }] */

const fooEmpty = /foo/;

const fooEmptyRegexp = new RegExp('foo');

const foo = /foo/v;

const fooRegexp = new RegExp('foo', 'v');
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Examples of correct code for this rule with the { "requireFlag": "u" } option:

/*eslint require-unicode-regexp: ["error", { "requireFlag": "u" }] */

const foo = /foo/u;

const fooRegexp = new RegExp('foo', 'u');
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requireFlag: “v”

The v flag may be a better choice when it is supported because it has more features than the u flag (e.g., the ability to test Unicode properties of strings). It does have a stricter syntax, however (e.g., the need to escape certain characters within character classes).

Examples of incorrect code for this rule with the { "requireFlag": "v" } option:

/*eslint require-unicode-regexp: ["error", { "requireFlag": "v" }] */

const fooEmpty = /foo/;

const fooEmptyRegexp = new RegExp('foo');

const foo = /foo/u;

const fooRegexp = new RegExp('foo', 'u');
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Examples of correct code for this rule with the { "requireFlag": "v" } option:

/*eslint require-unicode-regexp: ["error", { "requireFlag": "v" }] */

const foo = /foo/v;

const fooRegexp = new RegExp('foo', 'v');
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When Not To Use It

If you don’t want to warn on regular expressions without either a u or a v flag, then it’s safe to disable this rule.

Note on i flag and \w

In some cases, adding the u flag to a regular expression using both the i flag and the \w character class can change its behavior due to Unicode case folding.

For example:

const regexWithoutU = /^\w+$/i;
const regexWithU = /^\w+$/iu;

const str = "\u017f\u212a"; // Example Unicode characters

console.log(regexWithoutU.test(str)); // false
console.log(regexWithU.test(str)); // true
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If you prefer to use a non-Unicode-aware regex in this specific case, you can disable this rule using an eslint-disable comment:

/* eslint-disable require-unicode-regexp */
const regex = /^\w+$/i;
/* eslint-enable require-unicode-regexp */
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Version

This rule was introduced in ESLint v5.3.0.

Further Reading

GitHub - tc39/proposal-regexp-v-flag: UTS18 set notation in regular expressions
github.com

RegExp v flag with set notation and properties of strings · V8
v8.dev

Resources

• Rule source
• Tests source