Parsing a syslog file

[snippsat]

Do a test to make sure username is in line,as your regex dos not test that.
Use enumerate and not range(len(data)).
Then it will be like this.

import re
import json
from ast import literal_eval

filename = "systemfile.log"
# strip unneeded text from json format and save audit lines only
re_line= re.compile("audit")
data = []
with open(filename, "r") as in_file:
    # Loop over each log line
    for line in in_file:
        if re_line.search(line):
            data.append(line)

#print(data)

for index,line in enumerate(data):
    if 'username' in line:
        res = data[index].strip().split('[audit] ')[1]
        result = literal_eval(res)
        #print(result)
        timestamp = result['timestamp']
        user = result['user']['username']
        print(timestamp)
        print(user)

Output:
1633848664540
admin
1633848914062
admin