Beefy Boxes and Bandwidth Generously Provided by pair Networks
go ahead... be a heretic
 
PerlMonks  

Re^2: Perl Complains of Nested Quantifiers

by Kenosis (Priest)
on May 22, 2012 at 20:01 UTC ( #971864=note: print w/ replies, xml ) Need Help??


in reply to Re: Perl Complains of Nested Quantifiers
in thread Perl Complains of Nested Quantifiers

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.


Comment on Re^2: Perl Complains of Nested Quantifiers
Download Code
Replies are listed 'Best First'.
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

Re^3: Perl Complains of Nested Quantifiers
by AnomalousMonk (Abbot) 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; "
    (no output)

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://971864]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others musing on the Monastery: (6)
As of 2015-07-30 02:02 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    The top three priorities of my open tasks are (in descending order of likelihood to be worked on) ...









    Results (269 votes), past polls