What does "(? U)" in a regular expression do?

Question

I looked at how tokenization is implemented in scikit-learn and found this regex ( source ):

token_pattern = r"(?u)\b\w\w+\b" 

The regex is pretty simple, but I've never seen the (?u) part before. Can someone explain to me what this part does?

Answer 1

It includes the re.U ( re.UNICODE ) flag for this expression.

From the documentation:

(?iLmsux)

(one or more letters from the set of 'i' , 'L' , 'm' , 's' , 'u' , 'x' .) The group corresponds to an empty string; letters set the corresponding flags: re.I (ignore case), re.L (depends on the locale), re.M (multi-line), re.S (the dot matches all), re.U (depends on Unicode) and re.X (verbose), for the entire regular expression. (The flags are described in the section entitled “Module Content”.) This is useful if you want to include flags as part of a regular expression, instead of passing the flag argument to the re.compile() function.