import re

def test_regular_expression(regex, test_string) :
    pattern = re.compile(r'' + regex )
    match   = pattern.match(test_string)
    if match :
        try :
            return match.group(1)
        except :
            print('Match found but no substring returned')
            return ''
    else:
        print(regex, 'does not match', string)
        return ''

auth_log_line = 'Mar 16 11:58:13 users20 sshd[12041]: Accepted password for users257 from 65.96.149.57 port 60695 ssh2'

regex_time = '(d+:d+:d+)'
print('regex_time', regex_time, 't returned ', test_regular_expression(regex_time, auth_log_line))

Traceback (most recent call last):
  File "./test_code.py", line 24, in <module>
    print('regex_time', regex_time, 't returned ', test_regular_expression(regex_time, auth_log_line))
  File "./test_code.py", line 17, in test_regular_expression
    print(regex, 'does not match', string)
NameError: name 'string' is not defined