Re: Dominus on the non-greedy version of the ? quantifier

My take on regexp greediness, is that this affects the order in which matches are tried.

The greedy form of * and + try the longest match first, and successively decreasing the candidate match. Non-greedy forms *? and +? start with the shortest match and gradually increase the length.

The ? operator matches 0 or 1 occurrence. 1 occurrence is tried first. The ?? operator tries 0 occurrences first.

In the example you have given, it does not matter which is tried first, as you will get the same result. If there were another quantifier downstream of the ??, this could make a difference, as it would see the string to the left of the clause matching the ?? first, wherease a greedy ? would present the remainder of the string to the right of the match, first.

--

Oh Lord, won’t you burn me a Knoppix CD ?
My friends all rate Windows, I must disagree.
Your powers of persuasion will set them all free,
So oh Lord, won’t you burn me a Knoppix CD ?
(Missquoting Janis Joplin)

Comment on Re: Dominus on the non-greedy version of the ? quantifier

Replies are listed 'Best First'.
Re^2: Dominus on the non-greedy version of the ? quantifier by Aristotle (Chancellor) on Aug 07, 2006 at 09:22 UTC
I know what the quantifiers mean and how greediness works – which is exactly why I’m asking. I cannot see what Dominus is hinting at by bringing up this particular example, because greediness does not affect its result in any way. I was hoping someone would have an idea as to what he might have been thinking. Makeshifts last the longest.	[reply]
Re^3: Dominus on the non-greedy version of the ? quantifier by jhourcle (Prior) on Aug 07, 2006 at 14:19 UTC
...because greediness does not affect its result in any way. I was hoping someone would have an idea as to what he might have been thinking. You're only thinking about the final outcome. Sometimes, changes are not made for the outcome, but because it changes how it performs, and one method may be better for the normally expected input. Unfortunately, in this case, I'm not seeing that on my system the non-greedy being significantly better at matching the short version, and it's significantly worse at matching the long version: Read more... (868 Bytes) ~~Oh -- and it _does_ change the values of $1, $2, $3, but we have no reason from the code snippet given that they're important.,/strike>~~ Update: Removed extra comma, resulting in sloppy english grammer, and stressed the 'not'. Was : Unfortunately, in this case, I'm not seeing that on my system, the non-greedy being significantly better at matching the short version ... Update: tye is correct in the lack of difference. Why a non-greedy expression would match 1 when it has the option of matching 0 makes no sense to me, though.	[reply] [d/l]
Re^4: Dominus on the non-greedy version of the ? quantifier (diff) by tye (Sage) on Aug 07, 2006 at 17:38 UTC
the non-greedy being significantly better at matching the short version You consider 2% "significant"? 20% is rarely significant when it comes to Benchmark results, IME. Oh -- and it _does_ change the values of $1, $2, $3 It does? I don't see how. Testing with "-f" and "-field" don't show any difference for me. - tye	[reply]