String matches does not work

Hello! 

First of all, thanks for the lib. It's really useful. However, I can't make this template work

```vtl
#if( $limit != '' && $limit.matches('^\d$'))
{ "ok": 1 }
#else
{ "ok": 2 }
#end
```

The payload is 

```
{
  "limit": "3"
}
```

Isn't the `matches` function a built-in method? I'd love to help to implement this workaround if you instruct me how to do it. 

Thanks