Re: Perl Complains of Nested Quantifiers

The 'atomic' extended pattern "(?>pattern)" (available in 5.8) will give you possessiveness around the entire pattern or any sub-pattern of your choosing. See Extended Patterns in perlre. (The following example doesn't really illustrate atomic/possessive matching; for a good discussion and pertinent examples, see the docs.)

>perl -wMstrict -le
"print qq{perl ver. $]};
 my $rx = qr{ ((?> \d+)) }xms;
 'abc1234def' =~ $rx;
 print qq{'$1'};
"
perl ver. 5.008009
'1234'
[download]

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Re^2: Perl Complains of Nested Quantifiers by Kenosis (Priest) on May 22, 2012 at 20:01 UTC
The Extended Patterns documentation mentions that `"(?>pattern)` does not disable backtracking altogether once it has matched." A concern was expressed about "catastrophic backtracking." I'm curious about the nature of the 'catastrophe,' and whether your solution would satisfactorily avert it.	[reply] [d/l]
Re^3: Perl Complains of Nested Quantifiers by demerphq (Chancellor) on May 22, 2012 at 21:38 UTC
Well, what this means is that a pattern like /foo(?:baz)++baz/ will never match. If you had a string like "foobazbazbaz" it would fail because `(?:baz)++` will "gobble up" all of the "baz" in the string, and then refuse to give anything back. What it means by "does not disable backtracking" is that "foobazbazbaz foobazbazbaz" will attempt the /(?:baz)++/ twice, once after each "foo". Neither will match of course, and if the RE was _really_ smart it would know it could never match and wouldn't try at all, but it isn't. :-) --- $world=~s/war/peace/g	[reply] [d/l]
Re^3: Perl Complains of Nested Quantifiers by AnomalousMonk (Archbishop) on May 22, 2012 at 23:14 UTC
A concern was expressed about "catastrophic backtracking." I'm curious about ... whether your solution would satisfactorily avert it. It would not. AFAIU, possessive quantifiers like `++ + ?+ {`n,m`}+` are just special, limited cases of the general `(?>...)` atomic grouping. See example below. `>perl -wMstrict -le "my $s = 'foobazbazbaz foobazbazbaz'; print 'match 1' if $s =~ m{ foo (?> (?:baz)+) baz }xms; print 'match 2' if $s =~ m{ foo (?> . baz) baz }xms; "` [download] `(no output)`	[reply] [d/l] [select]