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eslint/require-unicode-regexp Pedantic

🚧 An auto-fix is planned for this rule, but not implemented at this time.

What it does

Enforce the use of u or v flag on regular expressions.

Why is this bad?

RegExp u flag has two effects:

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

/^[👍]$/.test("👍") //→ false
/^[👍]$/u.test("👍") //→ 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 doesn’t throw the error. It matches strings such as "a{1, 2" instead. Such a recovering logic is defined in Annex B.

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

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 ✅

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

Examples

Examples of incorrect code for this rule:

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:

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", "gv");

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

Configuration

requireFlag

type: "u" | "v"

default: null

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:

const fooEmpty = /foo/;
const fooEmptyRegexp = new RegExp("foo");
const foo = /foo/v;
const fooRegexp = new RegExp("foo", "v");

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

const foo = /foo/u;
const fooRegexp = new RegExp("foo", "u");

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:

const fooEmpty = /foo/;
const fooEmptyRegexp = new RegExp("foo");
const foo = /foo/u;
const fooRegexp = new RegExp("foo", "u");

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

const foo = /foo/v;
const fooRegexp = new RegExp("foo", "v");

How to use

To enable this rule using the config file or in the CLI, you can use:

Config (.oxlintrc.json)

{
  "rules": {
    "require-unicode-regexp": "error"
  }
}

Config (oxlint.config.ts)

import { defineConfig } from "oxlint";

export default defineConfig({
  rules: {
    "require-unicode-regexp": "error",
  },
});

CLI

oxlint --deny require-unicode-regexp

Version

This rule was added in v1.63.0.

References

• Rule Source
• Open rule in Oxc Playground