http://www.perlmonks.org?node_id=1030569

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

Though running this program, v1.pl:

( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( '' ) ) ))) ) ) ) ) ) ))))) ) =~("\(". ( ( '?')).'{' . ( ( '`')|'%') .('['^ ( '-') ) . ( '`'|'!') . ('`'|"\,"). '"'.('['^"\+").( '['^')').('`'|(')')).( '`'|'.').('['^'/').(('{')^ '[').'\\'.'"'.('`'|"\(").( '`'|'%').('`'|',').("\`"| ',').('`'|'/').('{'^'['). ('['^',').('`'|'/').("\["^ ')').('`'|',').('`'|('$')). '\\'.'\\'.('`'|'.').'\\'.'"' .';'.('!'^'+').'"'.'}'.')');
works fine with perl 5.16, running it with perl 5.17.10 produces an error:
Eval-group not allowed at runtime, use re 'eval' in regex m/(?{eval"pr +int\"hello world\\n\"; "})/ at v1.pl line 25.

Note that the Acme::EyeDrops-generated string being eval'ed above is a constant expression, as can be seen by running:

perl -MO=Deparse v1.pl
which produces:
'' =~ /(?{eval"print \"hello world\\n\";\n"})/; v1.pl syntax OK
Note further that running the deparsed program above works fine in perl 5.17 without requiring a leading use re 'eval'.

Though v1.pl can be made to work with perl 5.17.10 by adding a leading:

use re 'eval';
this destroys the artistic effect of having no alphanumeric characters at all. :) Moreover, I wonder why this is necessary, given the string being eval'ed is a constant expression and given that the deparsed program does not require it.

This change in use re 'eval' behavior was presumably introduced by Dave Mitchell's "re_eval jumbo fix" (see also ticket #113682). Given my test program above does not use tieing or overloading and no variables are interpolated in the (constant) expression, I wonder if this is a Perl 5.17 bug -- or am I missing something?

Assuming that use re 'eval' is really required for Perl v5.17.1+, I will need to fix the Acme::EyeDrops test suite to pass with the upcoming perl 5.18. Though adding a leading:

use if $] >= 5.017, re => 'eval';
or:
use if $^V && $^V gt 5.17.0, re => 'eval';
both seem to work fine, which is preferred?

References