Enforce the use of u flag on RegExp (require-unicode-regexp)

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

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.

Therefore, the u flag lets us work better with regular expressions.

Rule Details

This rule aims to enforce the use of u 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")

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")

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

When Not To Use It

If you don’t want to notify regular expressions with no u flag, then it’s safe to disable this rule.

Version

This rule was introduced in ESLint 5.3.0.

Resources

• Rule source
• Documentation source