in reply to A regex question.

Because you're using the  /g regex modifier for all matches, the string match position never resets to the start of the string, but advances for each and every match performed. (Update: I should have written: advances for each and every successful match performed.) 

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'xx111yy22xx333yy44zz555xx666';
 ;;
 print qq{first capture: '$1'} if $s =~ m{ (\d+) }xmsg;
 ;;
 print qq{subsequent captures: '$1'} while $s =~ m{ (\d+) }xmsg;
"
first capture: '111'
subsequent captures: '22'
subsequent captures: '333'
subsequent captures: '44'
subsequent captures: '555'
subsequent captures: '666'

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The fact that the first match in the OPed code is a negative | negated match (so no action is taken) does not matter; it's still a  /g match against the same string. Eliminating the  /g modifier on the first match will cause string match position to be reset before subsequent matches (which must still use /g).

Note that use of the  /g modifier in an
    if ($string =~ m{ ... }xmsg) { ... }
statement is useless except as a way to introduce exactly the behavior you are seeing!

Update: A number of incremental updates made.

Give a man a fish:  <%-(-(-(-<

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Re^2: A regex question.
by Anonymous Monk on Jul 31, 2015 at 23:57 UTC

    Hm, well I really didn't understand that about how the match operator works so I guess that would explain it. I had assumed /g was pointless on the if after thinking about it, but the part I really couldn't figure out was why the negative match would affect the subsequent one in any way.

[reply]

      Hello FeistyLemur, and welcome to the Monastery!

      See the section “Global matching” in perlretut#Using-regular-expressions-in-Perl, and in particular note the following (emphasis added.):

      In scalar context, successive invocations against a string will have //g jump from match to match, keeping track of position in the string as it goes along....
      A failed match or changing the target string resets the position.... The current position in the string is associated with the string, not the regexp.

      Hope that helps,

      Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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[d/l]

        Yes, I understand now why I was seeing the behavior in question. Things are working as I intended in my script. I just wanted to understand what I was missing about the match operator. I didn't realize the position in the string was associated with the string and not the regexp.

        I got the answer I was looking for and understand the match operator better now, thanks.

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      ... the negative match ...

      It's important to understand that the negation is made on the result of the  /eth1\.\d{4}\@/gm match against the  $ethernet string. The match itself is actually successful! It's as if the
          if ($ethernet !~ /eth1\.\d{4}\@/gm) { ... }
      statement had been written as
          if ( ! ($ethernet =~ /eth1\.\d{4}\@/gm)) { ... }
      (note =~ vice !~). In fact, that's pretty much the way Perl sees the code: 

      c:\@Work\Perl\monks>perl -wMstrict -MO=Deparse,-p -le
"my $ethernet=`ip addr`;
 if ($ethernet!~/eth1\.\d{4}\@/gm){
     print qq{No vlans exist on device.\n};
     exit;
 }
"
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use strict 'refs';
(my $ethernet = `ip addr`);
unless (($ethernet =~ /eth1\.\d{4}\@/gm)) {
    print("No vlans exist on device.\n");
    exit;
}
-e syntax OK

      [download]

      I had assumed /g was pointless on the if ...

      Given what you were trying to do, it was pointless, but it was not without effect!

      Give a man a fish:  <%-(-(-(-<

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