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Re^2: Substitution with regex and memory consumption

by k-mx (Scribe)
on Mar 01, 2020 at 09:19 UTC ( [id://11113603]=note: print w/replies, xml ) Need Help??


in reply to Re: Substitution with regex and memory consumption
in thread Substitution with regex and memory consumption

Okay, thank you! Some assumptions, please correct me if i'm wrong:

  1. Prior 5.18.0, s/// will copy original only if one of these was set: $&, $`, $'. Interpreter will set global PL_sawampersand flag that can't be disabled later. m// is also affected by this flag.
  2. Between 5.18.0 and 5.20.0, Perl can track usage of mentioned variables separately and copy only requested part of string.
  3. Perl 5.20.0+, successful s/// match always changes string, so COW mechanism always had to copy original.

So, before 5.20 we have choice: avoid $&, $`, $' and use /p modifier to explicitly copy ${^*MATCH}. Now we can use $&, $`, $', m// don't suffer from PL_sawampersand anymore, but s/// will always copy original string, PL_sawampersand state doesn't matter, and nothing we can do with that.

  • Comment on Re^2: Substitution with regex and memory consumption

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Re^3: Substitution with regex and memory consumption
by dave_the_m (Monsignor) on Mar 01, 2020 at 22:47 UTC
    That's roughly it, yes.

    Dave.

[reply]

      Isn't this behavior incorrect? Why we must copy variable on each substitution?

      I think we can: respect PL_sawampersand and /p flag for substitution and have only one copy for string (last one) in current scope.

[reply]
        As always the devil is in the detail. Consider for example: 
        {
    $s = ".....X.....";
    $s =~ s/X/Y/;
    $x = eval q{ "[ $`$& $' ]" };
}

        [download]
        You can't rely on PL_sawampersand.

        Dave.

[reply]
[d/l]

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