import re
 
regex = r"\\(\w+(?:-\w+)?, \w+)"
strings = [
    '.\\\\Jens, Jensen\\\\Rechnungen\\\\Rechnungen 2020\\\\somefoldername',
    '.\\Harald, Hardraala\\Rechnungen 2017',
    '.\\A - H\\Johan, Johanson\\Rechnungen 2017',
    '.\\\\Jens-Haudraf, Johan\\\\Rechnungen\\\\Rechnungen 2020\\\\anotherfoldername',
    '.\\A - H\\Funke, Felix'
]
 
for s in strings:
    matches = re.search(regex, s)
    if matches:
        print(matches.group(1))

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