> For the complete documentation index, see [llms.txt](https://dsheiko.gitbook.io/bycontract/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://dsheiko.gitbook.io/bycontract/custom-validators.md).

# Custom Validators

Basic type validators exposed exported as `is` object. So you can extend it:

```javascript
import { validate, is } from "bycontract";
is.email = function( val ){
  var re = /^(([^<>()[\]\\.,;:\s@"]+(\.[^<>()[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
  return re.test( val );
}
validate( "me@dsheiko.com", "email" ); // ok
validate( "bla-bla", "email" ); // ByContractError: expected email but got string
```