Only-data dataset loaded unexpectedly as validation split
The regex is needed as part of our effort to make datasets configurable without code. In particular we define some generic dataset repository structures that users can follow

> ```
> "[^a-zA-Z]*dev[^a-zA-Z]*"
> ```

unfortunately our glob doesn't support "^": 

https://github.com/fsspec/filesystem_spec/blob/3e739db7e53f5b408319dcc9d11e92bc1f938902/fsspec/spec.py#L465-L479
## 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`. 
 The regex is needed as part of our effort to make datasets configurable without code. In particular we define some generic dataset repository structures that users can follow

> ```
> "[^a-zA-Z]*dev[^a-zA-Z]*"
> ```

unfortunately our glob doesn't support "^": 

https://github.com/fsspec/filesystem_spec/blob/3e739db7e53f5b408319dcc9d11e92bc1f938902/fsspec/spec.py#L465-L479