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noMisleadingCharacterClass

src/content/docs/linter/rules/no-misleading-character-class.mdx

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import { Tabs, TabItem } from '@astrojs/starlight/components';

<Tabs> <TabItem label="JavaScript (and super languages)" icon="seti:javascript"> ## Summary - Rule available since: `v1.5.0` - Diagnostic Category: [`lint/suspicious/noMisleadingCharacterClass`](/reference/diagnostics#diagnostic-category) - This rule is **recommended**, meaning it is enabled by default. - This rule has a [**safe**](/linter/#safe-fixes) fix. - The default severity of this rule is [**error**](/reference/diagnostics#error). - Sources: - Same as [`no-misleading-character-class`](https://eslint.org/docs/latest/rules/no-misleading-character-class)

How to configure

json
{
	"linter": {
		"rules": {
			"suspicious": {
				"noMisleadingCharacterClass": "error"
			}
		}
	}
}

Description

Disallow characters made with multiple code points in character class syntax.

Unicode includes the characters which are made with multiple code points. e.g. Á, 🇯🇵, 👨‍👩‍👦. A RegExp character class /[abc]/ cannot handle characters with multiple code points. For example, the character ❇️ consists of two code points: ❇ (U+2747) and VARIATION SELECTOR-16 (U+FE0F). If this character is in a RegExp character class, it will match to either ❇ or VARIATION SELECTOR-16 rather than ❇️. This rule reports the regular expression literals which include multiple code point characters in character class syntax.

Examples

Invalid

js
/^[Á]$/u;
<pre class="language-text"><code class="language-text">code-block.js:1:4 <a href="https://biomejs.dev/linter/rules/no-misleading-character-class">lint/suspicious/noMisleadingCharacterClass</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">A character class cannot match a character and a combining character.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>/^[Á]$/u; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">A character and a combining character forms a new character. Replace the character class with an alternation.</span> </code></pre>
js
/^[❇️]$/u;
<pre class="language-text"><code class="language-text">code-block.js:1:4 <a href="https://biomejs.dev/linter/rules/no-misleading-character-class">lint/suspicious/noMisleadingCharacterClass</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">A character class cannot match a character and a combining character.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>/^[❇️]$/u; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">A character and a combining character forms a new character. Replace the character class with an alternation.</span> </code></pre>
js
/^[👶🏻]$/u;
<pre class="language-text"><code class="language-text">code-block.js:1:4 <a href="https://biomejs.dev/linter/rules/no-misleading-character-class">lint/suspicious/noMisleadingCharacterClass</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">A character class cannot match an emoji with a skin tone modifier.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>/^[👶🏻]$/u; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Replace the character class with an alternation.</span> </code></pre>
js
/^[🇯🇵]$/u;
<pre class="language-text"><code class="language-text">code-block.js:1:4 <a href="https://biomejs.dev/linter/rules/no-misleading-character-class">lint/suspicious/noMisleadingCharacterClass</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">A character class cannot match a pair of regional indicator symbols.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>/^[🇯🇵]$/u; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">A pair of regional indicator symbols encodes a country code. Replace the character class with an alternation.</span> </code></pre>
js
/^[👨‍👩‍👦]$/u;
<pre class="language-text"><code class="language-text">code-block.js:1:4 <a href="https://biomejs.dev/linter/rules/no-misleading-character-class">lint/suspicious/noMisleadingCharacterClass</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">A character class cannot match a joined character sequence.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>/^[👨‍👩‍👦]$/u; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">A zero width joiner composes several emojis into a new one. Replace the character class with an alternation.</span> </code></pre>
js
/^[👍]$/; // surrogate pair without u flag
<pre class="language-text"><code class="language-text">code-block.js:1:4 <a href="https://biomejs.dev/linter/rules/no-misleading-character-class">lint/suspicious/noMisleadingCharacterClass</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">A character class cannot match a surrogate pair. Add the 'u' unicode flag to match against them.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>/^[👍]$/; // surrogate pair without u flag <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">A surrogate pair forms a single codepoint, but is encoded as a pair of two characters. Without the unicode flag, the regex matches a single surrogate character.</span> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Safe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Add unicode </span><span style="color: lightgreen;"><strong>u</strong></span><span style="color: lightgreen;"> flag to regex</span> <strong> 1 │ </strong>/^[👍]$/<span style="color: MediumSeaGreen;">u</span>;<span style="opacity: 0.8;">·</span>//<span style="opacity: 0.8;">·</span>surrogate<span style="opacity: 0.8;">·</span>pair<span style="opacity: 0.8;">·</span>without<span style="opacity: 0.8;">·</span>u<span style="opacity: 0.8;">·</span>flag <strong> │ </strong> <span style="color: MediumSeaGreen;">+</span> </code></pre>

Valid

js
/^[abc]$/;
/^[👍]$/u;
/^[\q{👶🏻}]$/v;
</TabItem> </Tabs>