In the following code I want to get just the digits between '-' and 'u'.
I thought i could apply regular expression non capturing groups format (?: … )
to ignore everything from '-' to the first digit. But output always include it. How can i use noncapturing groups format to generate correct ouput?

```
df = pd.DataFrame(
    {'a' : [1,2,3,4], 
     'b' : ['41u -428u', '31u - 68u', '11u - 58u', '21u - 318u']
    })

df['b'].str.extract('((?:-[ ]*)[0-9]*)', expand=True)

```

[![enter image description here](https://i.stack.imgur.com/LG9IL.png)](https://i.stack.imgur.com/LG9IL.png)
[![enter image description here](https://i.stack.imgur.com/AYEwm.png)](https://i.stack.imgur.com/AYEwm.png)