example fails

```
const urns = require('urns');

console.log(urns.parseURN("example:a:b"))
```

```
Error: String "example:a:b" is not a valid RFC8141 compliant URN
    at Object.parseURN (/private/tmp/node_modules/urns/lib/parser.js:115:15)
    at Object.<anonymous> (/private/tmp/x.js:3:18)
```