Only-data dataset loaded unexpectedly as validation split
Yes, I had something like that on mind: "dev" not being part of a word.
```
"[^a-zA-Z]dev[^a-zA-Z]"
## 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`. 
 Yes, I had something like that on mind: "dev" not being part of a word.
```
"[^a-zA-Z]dev[^a-zA-Z]"