require-unicode-regexp

Enforce the use of u or v flag on RegExp

💡

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

RegExp u flag has two effects:

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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```
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:

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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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:

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")
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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)
}
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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.

Version

This rule was introduced in ESLint v5.3.0.

Resources

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