Re^17: Reusing a complex regexp in multiple spots, escaping the regexp

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
$
[download]

Dave.

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Re^18: Reusing a complex regexp in multiple spots, escaping the regexp by LanX (Saint) on Apr 17, 2026 at 15:13 UTC
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 `(??{...})` Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery} °) how popular are Perl builds without thread support?	[reply] [d/l] [select]
Re^19: Reusing a complex regexp in multiple spots, escaping the regexp by dave_the_m (Monsignor) on Apr 17, 2026 at 15:29 UTC
The "code injection" was just a quick and dirty demonstration of a side-effect. Its shows that on a threaded build, the expression which defines the pattern is assembled each time, but then thrown away unused apart from the first iteration. Think for example of a tied array. The following: `for my $i (0,1) { /abc$tied_array[$i]/; }` [download] will, on threaded builds, invoke both $tied_array->FETCH(0) and $tied_array->FETCH(1), but will discard the second value. Dave.	[reply] [d/l]
Re^20: Reusing a complex regexp in multiple spots, escaping the regexp by LanX (Saint) on Apr 17, 2026 at 17:07 UTC
Thanks for the better example! Does this double execution also effect officially embedded `(?{CODE})` ? Shouldn't this be just documented with /o or (in a better world) even better emitting a warning? I'm using side effects in regexes mainly for debugging. On a tangent: My system Perl seems to be with thread support, I dunno how often Perl is even installed or used without thread support. Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery}	[reply] [d/l]