regexEmptyAlternatives

Reports empty alternatives in regular expressions that may indicate a mistake.

✅ This rule is included in the ts logical and logicalStrict presets.

Empty alternatives in regular expressions match zero characters. While sometimes intentional, they often indicate a mistake such as a typo or incomplete pattern.

Examples

Empty Alternative at End

An empty alternative at the end of a group or pattern.

❌ Incorrect

const pattern = /a|/;

✅ Correct

const pattern = /a?/;

Empty Alternative at Start

An empty alternative at the start of a group or pattern.

❌ Incorrect

const pattern = /|a/;

✅ Correct

const pattern = /a?/;

Empty Alternative in Groups

Empty alternatives in capturing and non-capturing groups.

❌ Incorrect

const pattern = /(a|)/;

✅ Correct

const pattern = /(a)?/;

RegExp Constructor

The rule also checks regex patterns in RegExp constructor calls.

❌ Incorrect

const pattern = new RegExp("a|b|");

✅ Correct

const pattern = new RegExp("a|b");

Options

This rule is not configurable.

When Not To Use It

If you intentionally use empty alternatives to make a pattern optional and prefer this style over quantifiers, you might prefer to disable this rule. Some developers find (a|) more readable than a? for complex patterns.

Further Reading

• MDN: Disjunction
• MDN: Quantifier

Equivalents in Other Linters

• ESLint: regexp/no-empty-alternative