Rule Proposal: no-multi-code-point-character-in-character-class

[mysticatea]

Please describe what the rule should do:

If RegExp character class has multi-code-point character, it causes unexpected behavior, even if you are using u flag.

/^[❇️]$/u.test("❇️") //→ false

In above example, the ❇️ is made with two code points U+2747 and U+FE0F. JavaScript doesn't handle it as a character, so the character class matches to U+2747 or U+FE0F rather than the character. The U+FE00-FE0F is Variation Selectors.

Another well-known case is ZWJ U+200D. ZWJ joins multiple code points to a character.

/^[👨‍👩‍👦]$/u.test("👨‍👩‍👦") //→ false

So, this rule prevents characters which are made with multiple code points in character class syntax.

What category of rule is this? (place an "X" next to just one item)

[X] Warns about a potential error

Provide 2-3 code examples that this rule will warn about:

/[👍]/ // ERROR: a surrogate pair in a character class without u flag.
/[❇️]/u // ERROR: variation selectors in a character class.
/[👨‍👩‍👦]/u // ERROR: ZWJ in a character class.
/[🇯🇵]/u // ERROR: a U+1F1E6-1F1FF pair in a character class.
/[👶🏻]/u // ERROR: an emoji which is made with an emoji and skin tone selector, in a character class.

Why should this rule be included in ESLint (instead of a plugin)?

This is not library specific matter and it's difficult to distinguish whether character class syntax handles the character as one character or not.

[not-an-aardvark]

👍, but I feel like the rule name is a bit too long. How about something like no-invalid-character-classes or no-surrogate-pair-character-classes?

[mysticatea]

I agree. In fact, my first idea was no-destructuring-character-classes, but the word is used in destructuring assignment syntax.

no-invalid-character-classes is not bad.
no-surrogate-pair-character-classes sounds misleading since u flag can handle surrogate pairs correctly.

Hmm, how about no-dismantling-character-classes? The purpose of the rule is to report character classes which dismantle one character to multiple characters.

[platinumazure]

Marking as accepted.

I've also added "needs bikeshedding" since we need to agree on a name.