BUG: .str.split() method of pandas.Series  behaves strangely 
### 

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

- [ ] I have confirmed this bug exists on the master branch of pandas.


### Reproducible Example

```python
import pandas as pd 

series = pd.Series(['bae systems (international) ltd']) 

series.str.split('a. s.')  
# returns 
0    [b, stems (international) ltd]
```


### Issue Description

The .str.split() method of Pandas Series does not split items as expected. 


### Expected Behavior

The .str method of Pandas Series does not split items as expected. 

`s = "bae systems (international) ltd"`
`s.split('a.  s.') `

`# should return `
`['bae systems (international) ltd'] `

### Installed Versions

<details>

pandas 1.3.0 

</details>
