Re^15: Reusing a complex regexp in multiple spots, escaping the regexp

[dave_the_m]

Well for example, consider the following code:

for my $i (1..2) {
    print "match\n" if "x1" =~ /x${print "doing arg\n"; \1}/o;
}

On threaded perls it prints "doing arg" twice; on unthreaded, once.

Dave.

[LanX]

> on unthreaded, once.

I tried your code on "unthreaded" perl and it prints twice.

It seems like this weird "interpolation" ° resolves to "x1" =~ /x1/ which doesn't make much sense as a bug demo.

I don't understand how this is proving anything???

°) seems like you are dereferencing a ref to the literal number 1

  DB<1> say "${\1}"
1

you probably meant a the regex back-reference to the first match \1 ? But there is no first match ... ehm???

[dave_the_m]

I tried your code on "unthreaded" perl and it prints twice. It seems like this weird "interpolation" ° resolves to "x1" =~ /x1/ which doesn't make much sense as a bug demo.

On a perl interpreter compiled without threads support it prints "doing arg" once; on a threaded perl interpreter it prints twice - see the example run below. The match itself works correctly in both cases: it resolves to /x1/. My example was to show that there are strange side-effects which depend on the particlar perl interpreter you run it under. There are other edge case effects, the details of which I can't recall off the top of my head.

$ perl5420 -v
This is perl 5, version 42, subversion 0 (v5.42.0) built for x86_64-li
+nux
$ perl5420 ~/tmp/p
doing arg
match
match
$ perl5420t -v
This is perl 5, version 42, subversion 0 (v5.42.0) built for x86_64-li
+nux-thread-multi
$ perl5420t ~/tmp/p
doing arg
match
doing arg
match
$

Dave.

[LanX]

Thanks for the clarification. °

Honestly, when injecting code evaluation into a variable interpolation, I'm not surprised to get inconsistent side effects.

It would be of bigger concern if it also happened with more "official" means like (?{...}) or (??{...})

°) how popular are Perl builds without thread support?

[LanX]

Thanks!

Please, how can I reproduce this?

There are multiple threaded Perl models.

I never wrote threaded Perl¹, so please explain

• What's the output without /o then?

And as follow ups

• How does this qualify as a bug and not a feature?
• Is state better in this respect? °
• Shouldn't this be documented as side effect, (at least with a footnote)?

Footnotes

°) updated

¹) I suppose only a tiny minority ever did.

[choroba]

I think "threaded perl" means an interpreter compiled with -Dusethreads. It changes the output regardless of whether you use threads in the code or not.