Extracts all regex matches in a file and output to another file

re_extractor.py

#!/usr/bin/python3.5

import re, sys

SEPARATOR = '\n'

def extract(pattern, in_file_name, out_file_name):
    in_file_content = ""
    with open(in_file_name, 'r') as in_file:
        in_file_content = in_file.read()
    matches = re.findall(pattern, in_file_content)
    with open(out_file_name, 'w') as out_file:
        out_file.write(SEPARATOR.join(matches))

pattern = r"regex expression here"

extract(pattern, sys.argv[1], sys.argv[2])