Actually, part of the point of being precise in a regex is that it prevents pathological performance problems even in the face of nested quantifiers; even they can't get really slow. If the regex is "precise", then it can only match one way and it will never thrash trying a ton of different combinations, even if used as part of some larger regex. It also means that it won't surprise you by matching in some unexpected way.
The regex can backtrack out of what it tried to match but each step will find no alternative way to move forward and so it will backtrack out very directly. The use of (?>...) will only slightly speed up such a case.
How you verify that a regex is precise is you look at each decision point in the regex and verify that there is only one way to move forward from there. So if you have /A(B|C)*D/, after you've matched A your regex will try the following ways to move forward (in this order): 1) Try to match a longer version of A, 2) Match B, 3) Match C, 4) Match D.
If I'd not been in a bit of hurry when I'd posted, I would have done this and noticed my mistake. So let's look at the well-trodden territory of matching quotes when \" is used to esacpe the quote instead of "".
So in the case of /"((?:[^\\"]+|\\")*)"/, A is ", B is [^\\"]+, C is \\", and D is ". So A can't be matched in any longer way (since it has no quantifiers). C can only match if the next character is a backslash. D can only match if the next character is a double quote. Finally, B can only match if the next character is neither \ nor ", so that decision point is unambiguous. Note that the use of + is important here. /"((?:[^\\"]*|\\")*)"/ (note the + was replaced with *) is not precise.
Some who have read Mastering Regular Expressions would advise you to avoid the nested quantifiers and instead use /"((?:[^\\"]|\\")*)"/. That version is also precise. The main problem with it is a Perl quirk that prevents a regex part from matching more than 32K times and so that regex will die if used to try to match a string of more than 32K characters inside quotes. I also guess that it would be slightly slower.
Now, the problem with /"((?:[^"]+|"")*)"/ is that C ("") and D (") leads to an ambiguity. If I'd bothered to test my own test case, I would have also seen this. But coming up with sufficient test cases is quite a challenge so I find it works better to also check that my regex is precise. So I should have posted:
/"((?:[^"]+|"")*)"(?!")/
which is precise but in one case must look at the next two characters before knowing which route must be taken.
I prefer to not use (?>...) because avoiding it "forces" me to ensure that my regex is precise. But (?>...) can be useful both for preventing pathological performance problems and for preventing some surprises (though I haven't seen it used enough to get a feel for how often it will lead to other types of surprises).
- tye
In reply to Re^3: Help with Double Double Quotes regular expression (imprecise)
by tye
in thread Help with Double Double Quotes regular expression
by mattford63
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