Q:

Extract different format date from a string in Python

I have several strings, and have identified some formats of date on them, and would like to recognize date on each string
an_2011_02_12_azar.mp3 ->this is yyyy_mm_dd
20121112_Marcel.mp3    ->this is yyyymmdd
cdani_270607.mp3       ->this is ddmmyy
lica_07_03_15.mp3      ->this is dd_mm_yy

to do so I have:
foo = """
an_2011_02_12_azar.mp3
20121112_Marcel.mp3   
cdani_270607.mp3     
lica_07_03_15.mp3  
"""
try:
    lines = foo.split('\n')
except AttributeError:
    lines = x
for line in lines:
     print(line)
     #deals with 2011_02_12 format
     match = re.search(r'\d{4}_\d{2}_\d{2}', line)
     date = datetime.datetime.strptime(match.group(), '%Y_%m_%d').date()
     print(date)

How to apply several regular expressions so it can recognize dates? 

A:

If you remove the underscores:
datestr = line.replace('_', '')

then there would be only two date formats to deal with: yyyymmdd or ddmmyy.
Furthermore, every date string would consist of 6 to 8 digits which you could
find using the regex pattern r'\d{8}|\d{6}':
datestr = re.search(r'\d{8}|\d{6}', datestr).group()

The datestr could then be parsed with either
date = DT.datetime.strptime(datestr, '%d%m%y')

or 
date = DT.datetime.strptime(datestr, '%Y%m%d')

The pattern r'\d{8}|\d{6}' would also capture some possibly non-date-like strings, such digits which represent invalid dates. We could deal with those cases by using try..except to catch ValueErrors.

import re
import datetime as DT

foo = """\
an_2011_02_12_azar.mp3
20121112_Marcel.mp3   
cdani_270607.mp3     
lica_07_03_15.mp3  
an_2011_13_12_azar.mp3
"""

for line in foo.splitlines():
    datestr = line.replace('_', '')
    datestr = re.search(r'\d{8}|\d{6}', datestr).group()
    try:
        # %y matches 2-digit years
        date = DT.datetime.strptime(datestr, '%d%m%y')
    except ValueError:
        try:
            # %Y matches 4-digit years
            date = DT.datetime.strptime(datestr, '%Y%m%d')
        except ValueError:
            # handle the error case
            date = None
    print('{:23} --> {}'.format(line, date))

yields
an_2011_02_12_azar.mp3  --> 2011-02-12 00:00:00
20121112_Marcel.mp3     --> 2012-11-12 00:00:00
cdani_270607.mp3        --> 2007-06-27 00:00:00
lica_07_03_15.mp3       --> 2015-03-07 00:00:00
an_2011_13_12_azar.mp3  --> None

