in reply to Use of uninitialized value $1

Well, only if you 'use warnings', the compiler tells you that $1 has never been defined, and thus, can not be printed. If you want to capture "lib" in $1, then you need to use parenthesis in your regular expression. Like so:

use v5.10;
use warnings;
use strict;

my $var;
$var = "path_to_my_lib";
$var =~ m/(lib)/;
say $1;

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Which prints the captured "lib" (and gives no warnings).

$1, $2.. etc are weird variables. You can not assign them a value, and they disappear when they go out of scope, or a new regexp is encountered. You can solve it in a few ways, for example, test it before using it: 

if($1){
        say $1;
}

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or test the regular expression to match:

if( $var =~ m/(foo)/ ){
        say $1;
}else{
        say "no match found";
}

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Replies are listed 'Best First'.
Re^2: Use of uninitialized value $1
by AnomalousMonk (Archbishop) on Aug 25, 2016 at 12:29 UTC
    $1, $2.. etc are weird variables. ... they disappear when ... a new regexp is encountered. [Emphasis Added]

    Even more weird! These variables keep their value until a new regex is successfully matched or dynamic scope is exited. 

    c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $str = 'foo--bar';
 dd $1, $2;
 ;;
 $str =~ m{ (foo) .* (bar) }xms;
 print qq{A:  \$1 '$1'  \$2 '$2'};
 ;;
 $str =~ m{ (x) .* (y) }xms;
 print qq{B:  \$1 '$1'  \$2 '$2'};
"
(undef, undef)

A:  $1 'foo'  $2 'bar'
B:  $1 'foo'  $2 'bar'

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    Give a man a fish:  <%-{-{-{-<

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      "These variables keep their value until a new regex is successfully matched or dynamic scope is exited."

      I'm not sure that is true. I remember a node that fairly recently talked about this, but I couldn't find it, only What's happening to my $1?.

      What I remember is that a failed match could in some circumstances modify $1. So $1 is only reliable if the match succeeds (can't rely upon it being what it was after the previous regex after a failed match). I also remember that this behavior is Perl version specific.

      I haven't run across this in my code as I do whatever I'm going to do with $1 right after the regex and have never written code that decides whether $1 changed or not. I only use $1 if the match succeeded.

      Yes, this is a nit, but Perl Monks is good about resolving these nits and details.

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        Yes, this is a nit ...

        Regular expressions, love them though I do, consist in a vast collection of nits IMHO.

        I'm not sure that [these variables keep their value] is true. I remember a node that fairly recently talked about this, but I couldn't find it ... What I remember is that a failed match could in some circumstances modify $1.

        Now that you mention it, I seem to remember something along those lines myself. Unfortunately, my Super Search-fu is no better than yours, and I could come up with no pertinent node. Have to keep searching, searching... (Maybe it had to do with the match portion of  s/// having the same effects on regex variables as  m// does?)

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Re^2: Use of uninitialized value $1
by Pazitiff (Novice) on Aug 25, 2016 at 12:27 UTC
    Thanks! But what if i want capture the left word ("path_to_my_") of "lib"? In general, I need a regular expression to get what is left of the "lib". How this can be done using $1, $2, $3 ? Thanks. Sorry, but i have no experience in Perl and RX.
[reply]

      The Devil is in the Details! Exactly what are you matching against, and exactly what do you want to match? 

      c:\@Work\Perl\monks>perl -wMstrict -le
"my $var = '%&!~ path_to_a_lib_of_my_lib';
 ;;
 $var =~ m{ (.*) lib }xms;
 print qq{A: '$1'};
 ;;
 $var =~ m{ (.*?) lib }xms;
 print qq{B: '$1'};
 ;;
 $var =~ m{ (\w*?) lib }xms;
 print qq{C: '$1'};
"
A: '%&!~ path_to_a_lib_of_my_'
B: '%&!~ path_to_a_'
C: 'path_to_a_'

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      Update: Please see perlre, perlretut, and perlrequick.

      Further Update: If the string consists entirely in something like 'path_to_a_lib_of_my_lib', see also the start- and end-of-string "anchors"  \A \z \Z (particularly  \A \z in this case) for further narrowing what you want to capture.

      Give a man a fish:  <%-{-{-{-<

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      my $v = '/path/to/module.pm';
$v =~ m{^(.*)/(.*)$};
say "($1) ($2)";

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      prints out:

      (/path/to) (module.pm)

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      Thus, if you want your module name hardcoded:

      my $modulepath;
if($v =~ m{^(.*)module.pm$}){
 $modulepath = $1;
}

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      additional note: usually you write m// with slashes, but I write it with m{} brackets. This is also valid and allows you to use / inside the regexp, without having to escape it with a backslash.

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