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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_username(identifier):
 """Checks if a string is a email adress or not."""

    pattern = re.compile('.+@\w+\..+')
    if pattern.match(identifier):
        try:
            user = User.objects.get(email=identifier)
        except:
            raise Http404
        else:
            return user.username
    else:
        return identifier