Filter it. Use two re, one the one you've made, the other the double
hyphen filter:
pat2 = re.compile('.*?\-\-.*?')
If the string matches this re, then the string is rejected, if it DOES
NOT match (i.e. pat2.match('blah') returns None, i.e. if not
pat2.match('blah')), then it is accepted.
btw: You don't care about small letter?