Only-data dataset loaded unexpectedly as validation split
I see two options:
1. drop the "dev" keyword since it can be considered too generic
2. improve the pattern to something more reasonable, e.g. asking for a separator before and after "dev"
```python
["*[ ._-]dev[ ._-]*", "dev[ ._-]*"]
```

I think 2. is nice. If we agree on this one we can even decide to require the separation for the other split keywords "train", "test" etc.
## Describe the bug
As reported by @thomasw21 and @lhoestq, a dataset containing only a data file whose name matches the pattern `*dev*` will be returned as VALIDATION split, even if this is not the desired behavior, e.g. a file named `datosdevision.jsonl.gz`.
Only-data dataset loaded unexpectedly as validation split 
 ## Describe the bug
As reported by @thomasw21 and @lhoestq, a dataset containing only a data file whose name matches the pattern `*dev*` will be returned as VALIDATION split, even if this is not the desired behavior, e.g. a file named `datosdevision.jsonl.gz`. 
 I see two options:
1. drop the "dev" keyword since it can be considered too generic
2. improve the pattern to something more reasonable, e.g. asking for a separator before and after "dev"
```python
["*[ ._-]dev[ ._-]*", "dev[ ._-]*"]
```

I think 2. is nice. If we agree on this one we can even decide to require the separation for the other split keywords "train", "test" etc.