Beefy Boxes and Bandwidth Generously Provided by pair Networks
There's more than one way to do things
 
PerlMonks  

Re: What perl operations will consume C stack space?

by hv (Parson)
on Feb 25, 2006 at 11:40 UTC ( #532761=note: print w/ replies, xml ) Need Help??


in reply to What perl operations will consume C stack space?

Here's about the simplest one I can come up with:

perl -wle '$n=32766; $_="a" x $n; /(ab*){$n}/'

On most implementations this will coredump; depending on stacksize, it may coredump for much lower $n.

See the #24274 regexp metabug for more detail.

Hugo


Comment on Re: What perl operations will consume C stack space?
Download Code
Replies are listed 'Best First'.
Re^2: What perl operations will consume C stack space?
by BrowserUk (Pope) on Feb 27, 2006 at 04:04 UTC

    Thankyou. This was extremely helpful.

    One further question. Do you know of any legitimate, non-error uses of heavily backtracking regexes that cannot be better expressed using less stack hungry varients?


    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.

      Sorry, I can't answer that without fuller definitions of "legitimate" and "cannot be better expressed" - which does my previous example fall foul of?

      Any pattern that repeats a "non-simple" expression will consume C-stack space on each repetition. At least 3 of the 4 bugs linked to the metabug #24274 arose from people solving real world tasks, and there are more in the bugs database that should also be linked to the metabug - more recent ones often involve searching for particular structures in a genome.

      Here's another common fragment that invokes the problem:

      $_ = sprintf q{"%s"}, "a" x 32768; /"((?:\\.|[^"])+)"/;
      though it consumes stack at only half the rate of the /(ab*)*/ variety.

      Hugo

        I'm probably missing something, but I don't see the circumstances under which /(ab*){32766}/ couldn't be replaced by /(ab+|a){32766}/ with the same outcome (except the core dump), but much less backtracking?

        Likewise, is there anything that /"((?:\\.|[^"])+)"/ would match that /"([^"]+)"/ wouldn't?


        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://532761]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others about the Monastery: (11)
As of 2015-07-30 11:50 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 (271 votes), past polls