${^MATCH} sometimes fails in target of substitution?

raygun has asked for the wisdom of the Perl Monks concerning the following question:

Hello wise monks,

I'm unsure why these two lines produce different output. In particular, I don't know why the ${^MATCH} in the first line fails to be set to the matched part of the regular expression.

perl -e '$x = "abcde";             $x =~ s/b../==${^MATCH}==/p;  print
+ "$x\n"'
perl -e '$x = "abcde"; $sub="b.."; $x =~ s/$sub/==${^MATCH}==/p; print
+ "$x\n"'
[download]

Perl v5.12.4 produces:

a====e
a==bcd==e
[download]

The perlre documentation gives no restrictions on contexts in which {^MATCH} can be used, saying only that it's an optimized synonym for $&. And in fact, if I change the first line to use $& instead of {^MATCH}, it behaves as expected. Thank you for any hints!

Comment on ${^MATCH} sometimes fails in target of substitution? Select or Download Code

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Re: ${^MATCH} sometimes fails in target of substitution? by dave_the_m (Monsignor) on Jun 16, 2013 at 18:34 UTC
It was a bug; fixed in 5.18.0. Dave.	[reply]
Re: ${^MATCH} sometimes fails in target of substitution? by space_monk (Chaplain) on Jun 16, 2013 at 17:38 UTC
perlre states: As a workaround for this problem, Perl 5.10.0 introduces ${^PREMATCH} , ${^MATCH} and ${^POSTMATCH} , which are equivalent to $` , $& and $' , except that they are only guaranteed to be defined after a successful match that was executed with the /p (preserve) modifier. Update: The problem you have documented is a bug in releases prior to 5.18.See Re: ${^MATCH} sometimes fails in target of substitution?. Before 5.18 it appears that ${^MATCH} may only be available AFTER the match/replace. Adding print $(^MATCH} to your both example prints 'bcd' e.g. `perl -e '$x = "abcde"; $x =~ s/b../==${^MATCH}==/p; prin +t "$x Match:${^MATCH}\n"' a====e Match:bcd perl -e '$x = "abcde"; $sub="b.."; $x =~ s/$sub/==${^MATCH}==/p; print + "$x Match:${^MATCH}\n"' a==bcd==e Match:bcd` [download] If you spot any bugs in my solutions, it's because I've deliberately left them in as an exercise for the reader! :-)	[reply] [d/l]
Re^2: ${^MATCH} sometimes fails in target of substitution? by ikegami (Patriarch) on Jun 17, 2013 at 08:16 UTC
If you're replacing, there was a successful match. This is a bug. Testing reveals this bug has been fixed: `$ perl -v \| grep 'This is' This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-li +nux-thread-multi $ perl -E'$_ = "abcde"; s/b../==${^MATCH}==/p; say' a====e $ perl -E'$_ = "abcde"; $sub="b.."; s/$sub/==${^MATCH}==/p; say' a==bcd==e` [download] `$ perl -v \| grep 'This is' This is perl 5, version 18, subversion 0 (v5.18.0) built for x86_64-li +nux-thread-multi $ perl -E'$_ = "abcde"; s/b../==${^MATCH}==/p; say' a==bcd==e $ perl -E'$_ = "abcde"; $sub="b.."; s/$sub/==${^MATCH}==/p; say' a==bcd==e` [download]	[reply] [d/l] [select]
Re^3: ${^MATCH} sometimes fails in target of substitution? by space_monk (Chaplain) on Jun 17, 2013 at 10:11 UTC
Thanks, I amended my answer to reflect your comment and the earlier one of dave_the_m. If you spot any bugs in my solutions, it's because I've deliberately left them in as an exercise for the reader! :-)	[reply]
Re: ${^MATCH} sometimes fails in target of substitution? by farang (Chaplain) on Jun 16, 2013 at 18:10 UTC
I think this has to do with optimization of the regexp. I may not be using terminology quite right, but Perl sees that the $sub variable can be interpolated, so the regexp is eval'd at compile time, thereby populating ${^MATCH}. That is, it is only after being eval'd once that ${^MATCH} gets the value from the search. The following code also forces a similar 'second eval'. `my $x = "abcde"; $x =~ s/b../sprintf "==${^MATCH}=="/pe; print "$x\n";` [download]	[reply] [d/l]