Re^2: Substitution with regex and memory consumption

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

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.
Between 5.18.0 and 5.20.0, Perl can track usage of mentioned variables separately and copy only requested part of string.
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

Replies are listed 'Best First'.
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]
Re^4: Substitution with regex and memory consumption by k-mx (Scribe) on Mar 08, 2020 at 08:16 UTC
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]
Re^5: Substitution with regex and memory consumption by dave_the_m (Monsignor) on Mar 08, 2020 at 11:51 UTC
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]