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noEmptyCharacterClassInRegex

src/content/docs/linter/rules/no-empty-character-class-in-regex.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: `v1.3.0` - Diagnostic Category: [`lint/correctness/noEmptyCharacterClassInRegex`](/reference/diagnostics#diagnostic-category) - This rule is **recommended**, meaning it is enabled by default. - This rule doesn't have a fix. - The default severity of this rule is [**error**](/reference/diagnostics#error). - Sources: - Same as [`no-empty-character-class`](https://eslint.org/docs/latest/rules/no-empty-character-class)

How to configure

json
{
	"linter": {
		"rules": {
			"correctness": {
				"noEmptyCharacterClassInRegex": "error"
			}
		}
	}
}

Description

Disallow empty character classes in regular expression literals.

Empty character classes don't match anything. In contrast, negated empty classes match any character. They are often the result of a typing mistake.

Examples

Invalid

js
/^a[]/.test("a"); // false
<pre class="language-text"><code class="language-text">code-block.js:1:4 <a href="https://biomejs.dev/linter/rules/no-empty-character-class-in-regex">lint/correctness/noEmptyCharacterClassInRegex</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">The regular expression includes this </span><span style="color: Tomato;"><strong>empty character class</strong></span><span style="color: Tomato;">.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>/^a[]/.test(&quot;a&quot;); // false <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;">Empty character classes don't match anything. </span> <span style="color: lightgreen;">If you want to match against </span><span style="color: lightgreen;"><strong>[</strong></span><span style="color: lightgreen;">, escape it </span><span style="color: lightgreen;"><strong>&#92;[</strong></span><span style="color: lightgreen;">. </span> <span style="color: lightgreen;">Otherwise, remove the character class or fill it.</span> </code></pre>
js
/^a[^]/.test("ax"); // true
<pre class="language-text"><code class="language-text">code-block.js:1:4 <a href="https://biomejs.dev/linter/rules/no-empty-character-class-in-regex">lint/correctness/noEmptyCharacterClassInRegex</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">The regular expression includes this </span><span style="color: Tomato;"><strong>negated empty character class</strong></span><span style="color: Tomato;">.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>/^a[^]/.test(&quot;ax&quot;); // true <strong> │ </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;">Negated empty character classes match anything. </span> <span style="color: lightgreen;">If you want to match against </span><span style="color: lightgreen;"><strong>[</strong></span><span style="color: lightgreen;">, escape it </span><span style="color: lightgreen;"><strong>&#92;[</strong></span><span style="color: lightgreen;">. </span> <span style="color: lightgreen;">Otherwise, remove the character class or fill it.</span> </code></pre>

Valid

js
/^a[xy]/.test("ay"); // true
js
/^a[^xy]/.test("ab"); // true
js
/^a\[]/.test("a[]"); // true
</TabItem> </Tabs>