You can split the string without re.
>>> filename = '2019-12-05_18:02:28.801656_104_1_1575549141338.jpg'
>>> filename.split('_')
['2019-12-05', '18:02:28.801656', '104', '1', '1575549141338.jpg']
It doesn't give you exactly what you want. You could take it a step further and split on multiple characters ['_' and '.'].
>>> import re
>>> re.split("[\._]",filename)
['2019-12-05', '18:02:28', '801656', '104', '1', '1575549141338', 'jpg']
You could go further and build a re to match the entire string
>>> re.match(r'^(\d+-\d+-\d+)_(\d+:\d+:\d+)\.\d+_(\d+)_\d+_(\d+)\.jpg$', filename).groups()
('2019-12-05', '18:02:28', '104', '1575549141338')
You could take that a step further and extract a datetime
>>> import datetime
>>> date, x, y = re.match(r'^(\d+-\d+-\d+_\d+:\d+:\d+\.\d+)_(\d+)_\d+_(\d+)\.jpg$', filename).groups()
>>> datetime.datetime.strptime(date, '%Y-%m-%d_%H:%M:%S.%f')
datetime.datetime(2019, 12, 5, 18, 2, 28, 801656)