---
url: /reference/Function.prefixRegex.md
---
# Function: prefixRegex()

* **Exported from**: `rolldown/filter`
* **Type**: (`str`: `string`, `flags?`: `string`) => `RegExp`

Constructs a RegExp that matches a value that has the specified prefix.

This is useful for plugin hook filters.

## Parameters

### str

`string`

the string to match.

### flags?

`string`

flags for the RegExp.

## Returns

`RegExp`

## Example

```ts
import { prefixRegex } from '@rolldown/pluginutils';
const plugin = {
  name: 'plugin',
  resolveId: {
    filter: { id: prefixRegex('foo') },
    handler(id) {} // will only be called for IDs starting with `foo`
  }
}
```