Python's regular expression help

[goldtech]

Hi,
Trying to start out with simple things but apparently there's some
basics I need help with. This works OK:
>>> import re
>>> p = re.compile('(ab*)(sss)')
>>> m = p.match( 'absss' )
>>> m.group(0)
'absss'
>>> m.group(1)
'ab'
>>> m.group(2)
'sss'
...
But two questions:

How can I operate a regex on a string variable?
I'm doing something wrong here:

>>> f=r'abss'
>>> f
'abss'
>>> m = p.match( f )
>>> m.group(0)
Traceback (most recent call last):
  File "<pyshell#15>", line 1, in <module>
    m.group(0)
AttributeError: 'NoneType' object has no attribute 'group'

How do I implement a regex on a multiline string?  I thought this
might work but there's problem:

>>> p = re.compile('(ab*)(sss)', re.S)
>>> m = p.match( 'ab\nsss' )
>>> m.group(0)
Traceback (most recent call last):
  File "<pyshell#26>", line 1, in <module>
    m.group(0)
AttributeError: 'NoneType' object has no attribute 'group'
>>>

Thanks for the newbie regex help, Lee