regexLookaroundQuantifierOptimizations

Reports non-constant quantifiers in lookaround assertions that could be simplified.

✅ This rule is included in the ts logical presets.

Reports non-constant quantifiers in lookaround assertions that could be simplified. Lookahead and lookbehind assertions only check whether a pattern matches, not how much text it consumes. Non-constant quantifiers at the end of lookaheads or start of lookbehinds can be replaced with their minimum.

Examples

Lookahead with Zero Minimum

❌ Incorrect

const pattern = /(?=ba*)/;

✅ Correct

const pattern = /(?=b)/;

Lookahead with One Minimum

❌ Incorrect

const pattern = /(?=ba+)/;

✅ Correct

const pattern = /(?=ba)/;

Lookahead with Range Minimum

❌ Incorrect

const pattern = /(?=a{4,9})/;

✅ Correct

const pattern = /(?=a{4})/;

Lookbehind Assertions

Lookbehinds match from right to left, so non-constant quantifiers at the start are problematic.

❌ Incorrect

const pattern = /(?<=[a-c]*)/;

✅ Correct

const pattern = /(?<=)/;

Options

This rule is not configurable.

When Not To Use It

If you are intentionally using non-constant quantifiers in lookarounds for documentation purposes or if you are building patterns dynamically, you might want to disable this rule.

Further Reading

• MDN: Lookahead assertion
• MDN: Lookbehind assertion
• MDN: Quantifier

Equivalents in Other Linters

• ESLint: regexp/optimal-lookaround-quantifier