gistfile1.py

>>> a = re.compile(r'^[0-9_]+|\W|[_]+$')
>>> a.findall('__hello world')
['__', ' ']
>>> a.sub('', '[email protected]_table name hello')
'usernamegmailcom_tablenamehello'
>>> a.sub('', 'a')
'a'
>>> a.sub('', '_')
''
>>> a.sub('', '0')
''
>>> b = re.compile(r'\W')
>>> b.sub('', '[email protected]_table name hello')
'__usernamegmailcom_tablenamehello'