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

Given: 

while(<>) {
    s/(\d),(?=\d)/$1|$2/g;
    print;
}

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This triggers a warning of Use of uninitialized value in concatenation (.) or string whenever the regex matches.

Which I don't comprehend:
$_ is defined
$1 and $2 are both defined when the match occurs.
So why does the warning trigger?


Be Appropriate && Follow Your Curiosity

Replies are listed 'Best First'.
Re: regex match triggers "Use of unititialized value ..." warning
by BrowserUk (Patriarch) on Feb 22, 2011 at 02:30 UTC
    $1 and $2 are both defined when the match occurs.

    That can't be as there is only one set of capturing parens. The second set of parens are a non-capturing lookahead.

    This would probably do what you are trying to achieve: 

    s/(?<=\d),(?=\d)/|/g;

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    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]

      Hey future readers,
      If your Perl history has not involved look-ahead and look-behind regexes, this Q&A, especially BrowserUk's A, make for a nice learning opportunity.

      Well, nice if you consider my mis-use something that can be learned from.


      Be Appropriate && Follow Your Curiosity
[reply]
Re: regex match triggers "Use of unititialized value ..." warning
by hsinclai (Deacon) on Feb 22, 2011 at 02:34 UTC
    There is no $2, actually.
    So to clarify, sorry, the creation of a string: $1, then character |, then $2, fails.


    update: typo clarifications
[reply]
Re: regex match triggers "Use of unititialized value ..." warning
by toolic (Bishop) on Feb 22, 2011 at 02:32 UTC
    Show us a few lines of your input which trigger the warning message, preferably as a self-contained code sample using __DATA__: 
    while(<DATA>) {
    s/(\d),(?=\d)/$1|$2/g;
    print;
}
__DATA__

    [download]
[reply]
[d/l]

      I didn't include it, as I thought it was trivial.. 

      
#!/usr/bin/perl
use strict;
use warnings;

my $cnt;

while(<DATA>) {
    $cnt++;
    print "$cnt line\n";
    s/(\d),(?=\d)/$1|$2/g;
    print;
}

__DATA__
work with 19,43  or so
14,99
we have, at most, 14,23 to do though some say it is 18,44
Without digits, all the , will remain.
With digits 9, Ah ha! or
With,3 no change for a 2nd line
But again 3,4,5,6 all have bars.

      [download]


      Be Appropriate && Follow Your Curiosity
[reply]
[d/l]

      $2 doesn't exist. So that's cleared up now. Thank you. Removing $2 from the code removes the warnings, gives the expected output and all is well.


      Be Appropriate && Follow Your Curiosity
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