Python-сообщество

^[a-z\d]{1}[a-z\d]+[a-z\d]{1}$

>>> import re
>>> 
>>> def test_name(name):
...     return name != '' and '_' not in (name[0], name[-1]) and \
...            re.search(r'^[a-z\d_]+$', name) is not None
... 
>>> test_name('')
False
>>> test_name('_')
False
>>> test_name('_a')
False
>>> test_name('a_')
False
>>> test_name('a_a')
True
>>>

>>> import string
>>> 
>>> validset = set(string.ascii_lowercase + string.digits + '_')
>>> 
>>> def test_name(name):
...     return name != '' and '_' not in (name[0], name[-1]) and \
...            not set(name) - validset
... 
>>> test_name('')
False
>>> test_name('_')
False
>>> test_name('_a')
False
>>> test_name('a_')
False
>>> test_name('a_a')
True
>>>

import re
def test_name(name):
    return re.match(r'([a-z\d][a-z\d_]*[a-z\d]$)|([a-z\d]$)', name) is not None