Replace strings in a file using regular expressions

Question

How can I replace strings in a file with using regular expressions in Python?

I want to open a file in which I should replace strings for other strings and we need to use regular expressions (search and replace). What would be some example of opening a file and using it with a search and replace method?

Answer 1

# The following code will search 'MM/DD/YYYY' (e.g. 11/30/2016 or NOV/30/2016, etc ),
# and replace with 'MM-DD-YYYY' in multi-line mode.
import re
with open ('input.txt', 'r' ) as f:
    content = f.read()
    content_new = re.sub('(\d{2}|[a-yA-Y]{3})\/(\d{2})\/(\d{4})', r'\1-\2-\3', content, flags = re.M)

Answer 2

Here is a general format. You can either use re.sub or re.match, based on your requirement. Below is a general pattern for opening a file and doing it:

import re

input_file = open("input.h", "r")
output_file = open("output.h.h", "w")
br = 0
ot = 0

for line in input_file:
    match_br = re.match(r'\s*#define .*_BR (0x[a-zA-Z_0-9]{8})', line) # Should be your regular expression
    match_ot = re.match(r'\s*#define (.*)_OT (0x[a-zA-Z_0-9]+)', line) # Second regular expression

if match_br:
    br = match_br.group(1)
    # Do something

elif match_ot:
    ot = match_ot.group(2)
    # Do your replacement

else:
    output_file.write(line)