I have a file in which each line contains one timestamp as a part of that line. The timestamp format is 1996-07-04 00:00:00.0 . I want to convert this to 1996-07-04 00:00:00 without the millisecond in each line. I tried using re.sub() method in Python but it replaces it with the value given by me and does not retain the original timestamp.
I was using
re.sub("(\d\d\d\d-\d\d-\d\d\s+\d\d:\d\d:\d\d.\d)", "replace without millisec", cell)
The 2nd parameter is my problem.
You can use the following regex that will capture what you need to keep, and then use the backreference to restore it after a sub replacement:
\b(\d{4}-\d{2}-\d{2}\s+\d{2}:\d{2}:\d{2})\.\d+\b
Replace with \1.
See demo
IDEONE code:
import re
p = re.compile(r'\b(\d{4}-\d{2}-\d{2}\s+\d{2}:\d{2}:\d{2})\.\d+\b')
test_str = u"1996-07-04 00:00:00.0"
print re.sub(p, r"\1", test_str)
Note that you do not have to repeat the same subpatterns like \d\d\d\d, just use a limiting quantifier {n} where n is the number of times you need the subpattern to repeat. You can even set minimum and maximum boundaries like {1,4}, or just the minimum {2,}.
re.sub documentation: It is a backreference to the text captured by the first capturing group (the subpattern inside the first group of round brackets). You can even use \g<1> instead of just \1.
[:-2]?