What is wrong with this Python regular expression?

Question

Given a string, I want to find all the substrings consisting of two or three '4,'.

For example, given '1,4,3,2,1,1,4,4,3,2,1,4,4,3,2,1,4,4,4,3,2,' I want to get ['4,4,', '4,4,', '4,4,4'].

str_ = '1,4,4,3,2,1,1,4,4,3,2,1,4,4,3,2,1,4,4,3,2,'
m = re.findall(r"(4,){2,3}", str_)

what I get is : ['4,', '4,', '4,', '4,']

what's wrong?

It seems to me that the parenthesis wrapping '4,' is interpreted as grouping but not telling Python '4' and ',' should occur together. However, I don't know how to do this.

Solved, I should use the non-capturing version of regular parentheses — user3101491, Jul 31 '18 at 16:55
@pawamoy that won't work, the parenthesis plays a role of identifying the groups but binding characters together. We should use (?:4,) instead of (4,) — user3101491, Jul 31 '18 at 16:58
@user3101491 If you solved the issue, write it as an answer so it might help future readers. — Austin, Jul 31 '18 at 16:59

Andrej Kesely · Accepted Answer · 2018-07-31T17:22:47.707

1

Just use non-capturing group (online version of this regex here):

import re

s = '1,4,3,2,1,1,4,4,3,2,1,4,4,3,2,1,4,4,4,3,2,'

print(re.findall(r'(?:4,?){2,3}', s))

Prints:

['4,4,', '4,4,', '4,4,4,']

EDIT:

Edited regex to capture 2 or 3 elements "4,"

edited Jul 31 '18 at 17:22

answered Jul 31 '18 at 16:58

Andrej Kesely

81,807
10
31
56

1

`(?:(?:4,){1,2}4)` would handle the commas more precisely – TallChuck Jul 31 '18 at 17:46
Though then that could incorrectly match `4,4,44`. Maybe `(?<=^|,)((?:4,){1,2})(?=,|$)` then? – tripleee Jul 31 '18 at 17:53

What is wrong with this Python regular expression?

1 Answers1