insertString.py

import re

tester = "DIALER_STANDARD_AP1_2016-11-15.csv"
#tester = tester[:-14]
deck_name = re.search('_A(.+?)_2', tester).group(0)
deck_name = deck_name[1:] # remove the _ start
deck_name = deck_name[:-2] # remove the _2 end
deck_name = deck_name[:2] + "L" + deck_name[2:] # insert a string into the middle of another string
print deck_name