Change part of string in a file using regex in Python

Question

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 pyhton 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.

Answer 1

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,} .