[BUG] bids_validator.BIDSValidator returns incorrect evaluation of bids-like filenames

This is a real headache.... It seems that if the python version of the bids-validator returns `True` on some invalid BIDS file names, see:

```python
>>> from bids_validator import BIDSValidator
>>> validator = BIDSValidator()
>>> validator.is_bids('/sub_07/eeg/sub-07_task-matchingpennies_events.tsv')
True
>>>
```

Sigh... not sure where to go from here.