Re: BUG #17761: Questionable regular expression behavior
| От | Kosio Dimitrov |
|---|---|
| Тема | Re: BUG #17761: Questionable regular expression behavior |
| Дата | |
| Msg-id | 36522775.1746773.1675066286328@mail.yahoo.com обсуждение исходный текст |
| Ответ на | Re: BUG #17761: Questionable regular expression behavior (Tom Lane <tgl@sss.pgh.pa.us>) |
| Список | pgsql-bugs |
Yes, greediness seems to be the case,
One other solution I found to make it greedy is to add '.*$' at the end:
'(a).*?([1-9]\d*)\s*x\s*([1-9]\d*)(?:\s*x\s*([1-9]\d*))?.*$'
В петък, 27 януари 2023 г., 18:05:01 ч. Гринуич+2, Tom Lane <tgl@sss.pgh.pa.us> написа:
hubert depesz lubaczewski <depesz@depesz.com> writes:
> On Fri, Jan 27, 2023 at 09:27:35AM +0000, PG Bug reporting form wrote:
>> Executing:
>> select regexp_matches('a 1x1250x2500',
>> '(a).*?([1-9]\d*)\s*x\s*([1-9]\d*)(?:\s*x\s*([1-9]\d*))?');
>> returns: {a,1,1,NULL}
>> while executing:
>> select regexp_matches('a 1x1250x2500',
>> '(a|b).*?([1-9]\d*)\s*x\s*([1-9]\d*)(?:\s*x\s*([1-9]\d*))?');
>> returns: {a,1,1250,2500}
>>
>> Shouldn't both results be equal?
> The problem is, afair, that there is some state in pg's regexp engine
> that makes greedy/ungreedy decision once per regexp.
Yeah. Without having traced through it, I'm fairly sure that in the
first case, we have "(a)" which has no greediness, then ".*?" which
is non-greedy, and then that determines the overall greediness as
non-greedy, so it goes for the shortest overall match not the longest.
In the second case, "(a|b)" is greedy because anything involving "|"
is greedy, so we immediately decide we'll be greedy overall.
The fine manual explains how you can force greediness or non-greediness
when the engine's default rules for that don't do what you want.
regards, tom lane
> On Fri, Jan 27, 2023 at 09:27:35AM +0000, PG Bug reporting form wrote:
>> Executing:
>> select regexp_matches('a 1x1250x2500',
>> '(a).*?([1-9]\d*)\s*x\s*([1-9]\d*)(?:\s*x\s*([1-9]\d*))?');
>> returns: {a,1,1,NULL}
>> while executing:
>> select regexp_matches('a 1x1250x2500',
>> '(a|b).*?([1-9]\d*)\s*x\s*([1-9]\d*)(?:\s*x\s*([1-9]\d*))?');
>> returns: {a,1,1250,2500}
>>
>> Shouldn't both results be equal?
> The problem is, afair, that there is some state in pg's regexp engine
> that makes greedy/ungreedy decision once per regexp.
Yeah. Without having traced through it, I'm fairly sure that in the
first case, we have "(a)" which has no greediness, then ".*?" which
is non-greedy, and then that determines the overall greediness as
non-greedy, so it goes for the shortest overall match not the longest.
In the second case, "(a|b)" is greedy because anything involving "|"
is greedy, so we immediately decide we'll be greedy overall.
The fine manual explains how you can force greediness or non-greediness
when the engine's default rules for that don't do what you want.
regards, tom lane
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