RE-greedy or non greedy about '?'

RE-greedy or non greedy about '?' - Printable Version

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RE-greedy or non greedy about '?' - frank0903 - Jun-01-2020

Quote:The '*', '+', and '?' qualifiers are all greedy.
from https://docs.python.org/3/library/re.html

I think '?' qulifier is not greedy, the description is inaccurate, so anything else I don't know? or need to modify the part of doc?

RE: RE-greedy or non greedy about '?' - buran - Jun-01-2020

(Jun-01-2020, 04:17 AM)frank0903 Wrote: I think '?' qulifier is not greedy,

https://www.regular-expressions.info/refrepeat.html

RE: RE-greedy or non greedy about '?' - frank0903 - Jun-01-2020

import re
RE_QUESTION_MARK = 'yd?'
RE_ASTERISK = 'yd*'
test_str1 = 'ydddddd'
print(f'{test_str1} match result : {re.search(RE_ASTERISK, test_str1)}')
print(f'{test_str1} match result : {re.search(RE_QUESTION_MARK, test_str1)}')

Output:ydddddd match result : <re.Match object; span=(0, 7), match='ydddddd'>
ydddddd match result : <re.Match object; span=(0, 2), match='yd'>

according to the match results, '*' is greedy, acceptable. '?' only match 0 or 1 repetitions of the preceding RE, it's non-greedy. How can we call it greedy?

RE: RE-greedy or non greedy about '?' - buran - Jun-01-2020

greedy:
https://regex101.com/r/QmYBkp/1
from the explanations on the right-hand side:

Quote:"yd?" gm
y matches the character y literally (case sensitive)
d? matches the character d literally (case sensitive)
? Quantifier — Matches between zero and one times, as many times as possible, giving back as needed (greedy)

lazy:
https://regex101.com/r/qvKqqU/1
from the explanations on the right-hand side:

Quote:"yd??" gm
y matches the character y literally (case sensitive)
d? matches the character d literally (case sensitive)
? Quantifier — Matches between zero and one times, as few times as possible, expanding as needed (lazy)

RE: RE-greedy or non greedy about '?' - frank0903 - Jun-01-2020

@buran,Thanks! understood totally.
if anyone also have the same question, I hope this post will help to understand what's the real meaning of '?'
summary:
'?' quantifier matches between zero and one times, as many times as possible. In other words, there are two options, 0 times or 1 times; '?' quantifier selects '1 times', not '0 times', it means '?' is greedy, it wants more. I think that's the reason it's called greedy.
'??' quantifier matches between zero and one times, as few times as possible. In other words, '??' quantifier selects '0 times'.I think that's the reason it's called lazy or non-greedy.

RE: RE-greedy or non greedy about '?' - buran - Jun-01-2020

yep! and also if ? and * as quantifiers produce the same result (as implied by your example above), then probably one of them is not necessary