（[]）+和[] +有什么区别？

Question

>>> sentence = "Thomas Jefferson began building Monticello at the age of 26."
>>> tokens1 = re.split(r"([-\s.,;!?])+", sentence)
>>> tokens2 = re.split(r"[-\s.,;!?]+", sentence)
>>> tokens1 = ['Thomas', ' ', 'Jefferson', ' ', 'began', ' ', 'building', ' ', 'Monticello', ' ', 'at', ' ', 'the', ' ', 'age', ' ', 'of', ' ', '26', '.', '']
>>> tokens2 = ['Thomas', 'Jefferson', 'began', 'building', 'Monticello', 'at', 'the', 'age', 'of', '26', '']

Can you explain the purpose of ( and ) ?

Answer 1

(..) in a regex denotes a capturing group (aka "capturing parenthesis"). They are used when you want to extract values out of a pattern. In this case, you are using re.split function which behaves in a specific way when the pattern has capturing groups. According to the documentation:

re.split(pattern, string, maxsplit=0, flags=0)

Split string by the occurrences of pattern. If capturing parentheses are used in pattern, then the text of all groups in the pattern are also returned as part of the resulting list.

So normally, the delimiters used to split the string are not present in the result, like in your second example. However, if you use () , the text captured in the groups will also be in the result of the split. This is why you get a lot of ' ' in the first example. That is what is captured by your group ([-\\s.,;!?]) .

Answer 2

With a capturing group ( () ) in the regex used to split a string, split will include the captured parts.

In your case, you are splitting on one or more characters of whitespace and/or punctuation, and capturing the last of those characters to include in the split parts, which seems kind of a weird thing to do. I'd have expected you might want to capture all of the separator, which would look like r"([-\\s.,;!?]+)" (capturing one or more characters whitespace/punctuation characters, rather than matching one or more but only capturing the last).