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

Upgrading to 5.18 has caused "Use of uninitialized value in subtraction (-)..." to be printed when executing the following code:

while ($buf =~ /$pat/gp) {
    push @{ $self->{seen} }, [(pos $buf) - length ${^MATCH}, pos $buf]
        if not $sub or $sub->(substr ${^PREMATCH}, -200);
}

[download]

I'm certain that the uninitialized value it refers to is that of ${^MATCH}, but I can't for the life of me figure out why it thinks it's undefined.

The chunk of code falls within the context of a larger subroutine, and I've been unable to recreate the error message except during regular execution of the program of which it is a part.

The only sort of "use VERSION" declaration in the entire program is "require 5.010" in a base package. And the code worked fine before upgrading to 5.18. Any ideas?

Replies are listed 'Best First'.
Re: Upgrade to 5.18 causes uninitialized ${^MATCH}
by dave_the_m (Monsignor) on May 28, 2013 at 08:50 UTC
    I can reproduce it with the following code, which works in 5.16.x and broke in 5.18.0: 
    my $pat = qr/a/;
'aaaa'  =~ /$pat/gp or die;
print "MATCH=[${^MATCH}]\n";

    [download]
    Given that it was probably me who broke it, looks like I'll have to get it fixed for 5.18.1.

    Dave.

[reply]
[d/l]
      Now given ticket RT #118213

      Dave.

[reply]
      It appears to depend on qr. 
      my $pat = qr/a/p;
'aaaa'  =~ /$pat/g or die;
print "MATCH=[${^MATCH}]\n";

      [download]
      Appears to work OK on 5.18.0.
[reply]
[d/l]
[select]

      Yes, I overlooked the fact that $pat in my original example was pre-compiled with qr//.

[reply]
Re: Upgrade to 5.18 causes uninitialized ${^MATCH}
by AnomalousMonk (Archbishop) on May 28, 2013 at 02:24 UTC

    It's possible for either or both of pos or  ${^MATCH} to be undefined after a match, which is easily demonstrated (Perl version 5.14): 

    >perl -wMstrict -le
"my @strs = (qw(123x foo ***), '');
 ;;
 for my $s (@strs) {
   $s =~ m{ \d+ }xmsgp;
   printf qq{for '%s': pos == %s, \${^MATCH} eq '%s' \n}, $s,
     pos($s) // 'undefined', ${^MATCH} // 'undefined',
     ;
   }
"
for '123x': pos == 3, ${^MATCH} eq '123'
for 'foo': pos == undefined, ${^MATCH} eq 'undefined'
for '***': pos == undefined, ${^MATCH} eq 'undefined'
for '': pos == undefined, ${^MATCH} eq 'undefined'

    [download]

    Rather than Perl, I think the most likely explanation is that the data you are processing has changed.

    Update: ... and the length of an undef is undefined.

[reply]
[d/l]
[select]
Re: Upgrade to 5.18 causes uninitialized ${^MATCH}
by kcott (Archbishop) on May 28, 2013 at 03:00 UTC

    G'day nbtrap,

    Welcome to the monastery.

    I'm using 5.18.0. Without any context or knowledge of the values of $buf, $pat, $self or $sub, I am unable to reproduce your problem.

    pos() and ${^MATCH} are returning the values I would expect:

    $ perl -Mstrict -Mwarnings -E '
    my $buf = q{aXa};
    while ($buf =~ /a/gp) {
        say q{pos($buf):};
        say pos($buf);
        say q{${^MATCH}:};
        say ${^MATCH};
    }
'
pos($buf):
1
${^MATCH}:
a
pos($buf):
3
${^MATCH}:
a

    [download]

    perldelta (for 5.18.0) mentions a change to ${^MATCH} under Internal Changes which may be relevant.

    perldoc.perl.org hasn't updated to 5.18.0 yet. If you're using online documentation, I suggest using the CPAN perl 5.18.0 doco. (The commandline perldoc utility is up-to-date.)

    You may get some hints for creating a short, self-contained script that reproduces your problem from "How do I post a question effectively?".

    -- Ken

[reply]
[d/l]
[select]

      Thank you. I'm aware of the changes documented in the 5.18 manual pages, but none of them seemed to address this problem. And I know a working script would have been very helpful, but I couldn't get this to work outside the context of my rather large program, so I decided to cross my fingers and hope someone else might be experiencing the same thing.

[reply]

Back to Seekers of Perl Wisdom