import re
p=re.compile('S(.+?)N')
s='ASDFANSAAAAAFGNDASMPRKYN'
s1=p.findall(s)
print s1

['DFA', 'AAAAAFG', 'MPRKY']

'DFA'  length=3

>>> s1 = ['DFA', 'AAAAAFG', 'MPRKY']
>>> min(s1, key=len)
'DFA'

>>> s2 = ['foo', 'bar', 'baz', 'spam', 'eggs', 'knight']
>>> s2_min_len = len(min(s2, key=len))
>>> [e for e in s2 if len(e) is s2_min_len]
['foo', 'bar', 'baz']

import re
p=re.compile('S(.{2,6}?)N')
s='ASDFANSAAAAAFGNDASMPRKYNSAAN'
s1=p.findall(s)
if s1:
    print sorted(s1, key=len)[0]
    print min(s1, key=len) # as suggested by Nick Presta