Perl does not reevaluate regex variable

Question

I am in charge of upgrading this Perl script from Perl v5.6.1 (2001) to v5.20.2 (2015). I have these 2 regex variable:

foreach (@filelist) {
    chomp;
    my $File = $_;
    if ( $File =~ qr/.+/o ) {
        if ( $BaseLine ) {

            $BaseLineRegExpA = qr/^\Q$BaseLine\E\\/io; #these 2 regexes
            $BaseLineRegExpB = qr/^\Q$BaseLine\E;/io;  #these 2 regexes

            if ( $File =~ /$BaseLineRegExpA/ ) {
                #...

            } elsif ( (!($File =~ /$BaseLineRegExpB/)) && (!(lc( $File ) eq lc( $BaseLine ) )) ) {
                $BaseLine = $File;
            }
        }
     }
}

So, I have 2 questions:

In the old Perl version, the $BaseLineRegExpA and $BaseLineRegExpB gets reevaluate every time $BaseLine changes, but in the new Perl, it does not. How do I make it changes? I've tried my $BaseLineRegExpA, it still does not change.
In the old Perl, $BaseLineRegExpA evals to: (?i-xsm:^F:\\dd\\), and in the new Perl, it evals to (?^i:^F:\\dd\\). My questions is, is there a different between ?i-xsm:^ and ?^i:^?

Thanks so much, unfortunately, these are legacy scripts and I don't know much about Perl.

hobbs · Accepted Answer · 2015-06-03 03:16:21Z

7

The o modifier prevents re-evaluating variables substituted into regexes. It's curious that it didn't happen for you on 5.6, but it's probably because qr was still new in that version. Removing it (changing /io to /i) should make things work the way you expect.
The (?i-xsm) encodes the regex modifier flags that are in effect (i is turned on, x, s, and m are turned off). Sometime around perl 5.14, Perl got some new regex modifier flags, which would change the stringification of all regexes. Since this was already a backwards-incompatible change, it was decided to do it in a way that would limit the hassle caused by adding any new flags down the road, and so the ^ character was used to represent the "default" set of flags. So (?^i) means "the default flags, plus the i flag". They both mean basically the same thing, and there's nothing you should worry about.

answered Jun 3, 2015 at 3:16

hobbs

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2 Comments

ikegami Over a year ago

Tested. qr//o was definitely broken in 5.6. The o was ignored.

hobbs Over a year ago

@NamNgo you're welcome. Just as side note, I've been using Perl for 15 years and there isn't anyplace I would recommend using /o. At best it makes a very small performance improvement, easily outweighed by its potential for creating bugs.

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