NLTK TweetTokenizer vulnerable to denial of service through catastrophic regex backtracking

The URLS regular expression in nltk/tokenize/casual.py, compiled into TweetTokenizer.WORDRE and applied by TweetTokenizer.tokenize, contains a naked-domain branch whose domain-label prefix a-z0-9+?:.\-a-z0-9+ is unbounded. Input consisting of many alternating label separators can be partitioned i...