SyntaxError: nothing to repeat

The JavaScript exception "nothing to repeat" or "invalid quantifier in regular expression" occurs when a quantifier in a regular expression is applied to nothing or applied to an assertion.

Message#

SyntaxError: Invalid regular expression: /\b+/: Nothing to repeat (V8-based)
SyntaxError: Invalid regular expression: /(?=)+/u: Invalid quantifier (V8-based)
SyntaxError: nothing to repeat (Firefox)
SyntaxError: invalid quantifier in regular expression (Firefox)
SyntaxError: Invalid regular expression: nothing to repeat (Safari)

Error type#

{{jsxref("SyntaxError")}}

What went wrong?#

Quantifiers are used to specify how many times a character or group of characters can appear in a regular expression. For example, a{3} matches the character a exactly three times. Therefore, if the thing preceding the quantifier is not something that matches characters, the quantifier is invalid. For example: quantifiers at the start of a capturing group, at the start of a disjunction alternative, etc., cannot repeat anything. Assertions don't consume characters, so it also doesn't make sense to repeat them.

In Unicode-unaware mode, there's a deprecated syntax that allows the lookahead assertions to be quantified. This is a deprecated syntax and you should not rely on it.

Examples#

Invalid cases#

```js example-bad /\b+/; // \b is a word boundary assertion, it doesn't consume characters /(hello)/;

### Valid cases

```js example-good
/b+/; // b is a character, it can be repeated
/(\*hello\*)/; // Escape the asterisks to match them literally

See also#

• Regular expressions
• Quantifier: *, +, ?, {n}, {n,}, {n,m}