Python code to use a regular expression to make sure a string is alphanumeric plus . - _

re.match does not return a boolean; it returns a MatchObject on a match, or None on a non-match.

>>> re.match("^[a-zA-Z0-9_.-]+$", "hello")
<_sre.SRE_Match object at 0xb7600250>
>>> re.match("^[a-zA-Z0-9_.-]+$", "    ")
>>> print re.match("^[a-zA-Z0-9_.-]+$", "    ")
None

So, you shouldn't do re.match(...) == True; rather, you should be checking re.match(...) is not None in this case, which can be further shortened to just if re.match(...).

Never use == True or == False in a comparison. Many types already have a bool equivalent which you should use instead:

if re.match("^[a-zA-Z0-9_.-]+$", username):

Could also shorten it slightly to :

if re.match(r'^[\w.-]+$', username):