I'm new to Python and trying to analyse some data. So I've imported and concatenated all the csv files in a folder into a single dataframe. I'm trying to extract part of the file name to use as a header and after searching, I find that you'd normally use regex.

The filenames are like this: `'Varying Concentration2_20190712-145158_Base Media.csv'`, `'Varying Concentration2_20190712-145158_250 g per l.csv'`, etc
So the part I'm trying to extract is after the `_` and before the `.csv`. 

I've tried:

```py
for fname in all_data:
    res = re.findall("(?<=_)(\w+).csv$", fname)
    if not res: continue
    print (res)

```

and also `"(?<=[0-9]+_)(\w+)"`
but it does not seem to work. 

The desired output would be a list containing `'Base Media'`, `'150g per l`' and so on.