r in regular expression

 http://stackoverflow.com/questions/2241600/python-regex-r-prefix

 the 'r' means the the following is a "raw string".

ie. backslash characters are treated literally instead of signifying special treatment of the following character.

http://docs.python.org/reference/lexical_analysis.html#literals

so '\n' is a single newline
and r'\n' is two characters - a backslash and the letter 'n'
another way to write it would be '\\n' because the first backslash escapes the second

an equivalent way of writing this

print (re.sub(r'(\b\w+)(\s+\1\b)+', r'\1', 'hello     there      there'))

is

print (re.sub('(\\b\\w+)(\\s+\\1\\b)+', '\\1', 'hello     there      there'))

Because of the way Python treats characters that are not valid escape characters, not all of those double backslashes are necessary - eg '\s'=='\\s' however the same is not true for '\b' and '\\b'. My preference is to be explicit and double all the backslashes.