import re
text = (
    """@RayFranco is answering to @jjconti, this is a real '@username83' but this is an@email.com, and this is a @probablyfaketwitterusername"""
    )
ftext = re.sub('(^|[^@\\w])@(\\w{1,15})\\b',
    '\\1<a href="http://twitter.com/\\2">\\2</a>', text)
regex for Twitter username