正则表达式中的圆括号是什么意思？

Question

I don't understand why the regex ^(.)+$ matches the last letter of a string. I thought it would match the whole string.

Example in Python:

>>> text = 'This is a sentence'
>>> re.findall('^(.)+$', text)
['e']

Answer 1

If there's a capturing group (or groups), re.findall returns differently:

If one or more groups are present in the pattern, return a list of groups; this will be a list of tuples if the pattern has more than one group. Empty matches are included in the result unless they touch the beginning of another match.

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And according to MatchObject.group documentation :

If a group matches multiple times, only the last match is accessible :

If you want to get whole string, use a non-capturing group:

>>> re.findall('^(?:.)+$', text)
['This is a sentence']

or don't use capturing groups at all:

>>> re.findall('^.+$', text)
['This is a sentence']

or change the group to capturing all:

>>> re.findall('^(.+)$', text)
['This is a sentence']
>>> re.findall('(^.+$)', text)
['This is a sentence']

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Alternatively, you can use re.finditer which yield match objects. Using MatchObject.group() , you can get the whole matched string:

>>> [m.group() for m in re.finditer('^(.)+$', text)]
['This is a sentence']

Answer 2

Because the capture group is just one character (.) . The regex engine will continue to match the whole string because of the + quantifier, and each time, the capture group will be updated to the latest match. In the end, the capture group will be the last character.

Even if you use findall , the first time the regex is applied, because of the + quantifier it will continue to match the whole string up to the end. And since the end of the string was reached, the regex won't be applied again, and the call returns just one result.

If you remove the + quantifier, then the first time, the regex will match just one character, so the regex will be applied again and again, until the whole string will be consumed, and findall will return a list of all the characters in the string.

Answer 3

NOte that + is greedy by default which matches all the characters upto the last. Since only the dot present inside the capturing group, the above regex matches all the characters from the start but captures only the last character. Since findall function gives the first preference to groups, it just prints out the chars present inside the groups.

re.findall('^(.+)$', text)