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eslint/no-misleading-character-class Correctness

✅ This rule is turned on by default.

💡 A suggestion is available for this rule.

What it does

This rule reports regular expressions which include multiple code point characters in character class syntax. This includes:

• Characters with combining marks (e.g., Á where A is followed by a combining acute accent)
• Characters with emoji modifiers (e.g., 👶🏻)
• Pairs of regional indicator symbols (e.g., 🇯🇵)
• Characters joined by zero-width joiner (ZWJ) (e.g., 👨‍👩‍👦)
• Surrogate pairs without the Unicode flag (e.g., /^[👍]$/)

Why is this bad?

Unicode includes characters which are made by multiple code points. RegExp character class syntax (/[abc]/) cannot handle characters which are made by multiple code points as a character; those characters will be dissolved to each code point. For example, ❇️ is made by ❇ (U+2747) and VARIATION SELECTOR-16 (U+FE0F). If this character is in a RegExp character class, it will match either ❇ (U+2747) or VARIATION SELECTOR-16 (U+FE0F) rather than ❇️.

This can lead to regular expressions that do not match what the author intended, especially for emoji, regional indicators, and characters with combining marks.

Examples

Examples of incorrect code for this rule:

/^[Á]$/u;
/^[❇️]$/u;
/^[👶🏻]$/u;
/^[🇯🇵]$/u;
/^[👨‍👩‍👦]$/u;
/^[👍]$/;
new RegExp("[🎵]");

Examples of correct code for this rule:

/^[abc]$/;
/^[👍]$/u;
/[\u00B7\u0300-\u036F]/u;
new RegExp("^[\u{1F1EF}\u{1F1F5}]", "u");

Configuration

This rule accepts a configuration object with the following properties:

allowEscape

type: boolean

default: false

When set to true, the rule allows any grouping of code points inside a character class as long as they are written using escape sequences.

Examples of incorrect code for this rule with { "allowEscape": true }:

/[\uD83D]/; // backslash can be omitted
new RegExp("[\ud83d" + "\udc4d]");

Examples of correct code for this rule with { "allowEscape": true }:

/[\ud83d\udc4d]/;
/[\u00B7\u0300-\u036F]/u;
/[👨\u200d👩]/u;
new RegExp("[\x41\u0301]");
new RegExp(`[\u{1F1EF}\u{1F1F5}]`, "u");
new RegExp("[\\u{1F1EF}\\u{1F1F5}]", "u");

How to use

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

Config (.oxlintrc.json)

{
  "rules": {
    "no-misleading-character-class": "error"
  }
}

Config (oxlint.config.ts)

import { defineConfig } from "oxlint";

export default defineConfig({
  rules: {
    "no-misleading-character-class": "error",
  },
});

CLI

oxlint --deny no-misleading-character-class

Version

This rule was added in v1.17.0.

References

• Rule Source
• Upstream rule docs
• Open rule in Oxc Playground