python - 正则表达式中*的疑问

Question

import re
text = 'thenextnothingis123456'
print(re.search(r'(\d*)', text).group(0))

为什么这段代码不能匹配出数字？如果将d替换成w反倒可以匹配出所有字符？

Answer 1

Because the meaning of * in r'(d*)' is to match 0 or more, it can not match any characters. Use re.search to start matching from the beginning of the string. Because the first character of 'thenextnothingis123456' is not a number, it cannot be matched, but r'(d*)' can not match any characters, so an empty string is returned.

And r'(d+)' requires matching 1 to multiple numbers. When used to match 'thenextnothingis123456', it is found that the first character is not a letter, and it will continue to try the second character until character 1 starts with numbers. So "123456" is matched. You can understand it by looking at the output below.

>>> import re
>>> text = 'thenextnothingis123456'
>>> p = re.search(r'(\d*)', text)
>>> p.start()
0
>>> p.end()
0
>>> p.groups()
('',)
>>> p = re.search(r'(\d+)', text)
>>> p.start()
16
>>> p.end()
22
>>> p.groups()
('123456',)

Answer 2

Why can’t this code match numbers?

I feel that it has actually been matched, which is the number of 0 times: Your .group(0) did not report a nonetype error, indicating that the match was successful

Answer 3

30-minute introductory tutorial on regular expressions

Answer 4

>>> print(re.search(r'(\d*)', "abcd1234").group(0))  ########11111111

>>> print(re.search(r'(\d*)', "1234abcd").group(0))
1234
>>> print(re.search(r'[a-z]*(\d*)', "abcd1234").group(1))
1234

>>> print(re.search(r'(\d{0})', "abcd1234").group(0))  #######2222222

>>> print(re.search(r'(\d+)', "abcd1234xxx4321").group(0))
1234
>>> print(re.search(r'(\d+)', "abcd1234xxx4321").group(1))   ####可见re.search第一次匹配成功后就不再接着匹配
1234
>>> print(re.search(r'(\d+)', "abcd1234xxx4321").group(2))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: no such group

11111111 The result of this sentence is empty because re.search uses #######2222222

Answer 5

You try d, and * means 0 or more times

Answer 6

Answer 7

>>> import re
>>> text = 'thenextnothingis123456'
>>> print(re.search(r'(\d+)', text).group(0))
123456
>>>
可以用(\d+)规则匹配~
同问为什么r'(\d*)无法匹配出字符串"123456"？