>>> import re
>>> 
>>> regex = re.compile('(?:(?:"[a-z-0-9_!:,]*")|(?:[a-z-0-9_]|(?:\.(?!\.))+))+$')
>>> 
>>> regex.match('a"."b')
>>>

regex = re.compile(r'^("([a-z-0-9_!:,]|\.(?!\.))*"|[a-z-0-9_]|\.(?!\.))+$')

#!/usr/bin/env python3
 
import re
  
def is_login(s):
    y, n, quo, pt = True, 0, False, False
    for c in s:
        if not y or n == 128 or (c in '!,:' and not quo) or \
           (c == '.' and pt) or not re.search(r'^[a-z0-9"._!,:-]$', c):
               y = False
               break
        n += 1
        if c == '"':
            quo = not quo
            pt = False
        else:
            pt = c == '.'
    y = s != '' and y and not quo
    return y
 
if __name__ == '__main__':
    for s in [
        '',
        'abc',
        'a' * 128,
        'a' * 129,
        'abc..def',
        'abc"."def',
        '"abc!,:def"',
        'abc!,:def',
        '"abc!,:def""',
        '""',
        '""#'
        ]:
            print('<{}>: {}'.format(s, is_login(s)))

[guest@localhost login_ind]$ ./login_ind.py
<>: False
<abc>: True
<aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa>: True
<aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa>: False
<abc..def>: False
<abc"."def>: True
<"abc!,:def">: True
<abc!,:def>: False
<"abc!,:def"">: False
<"">: True
<""#>: False
[guest@localhost login_ind]$