Use of uninitialized value in pattern match (m//)

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

Hi there. I am attempting to test a script that worked without error on my windows box to a Red Hat Linux distro. I get the error 'Use of uninitialized value in pattern match (m//)' on the first line of the if statement from the function below. I've found examples of this error online but fail to see how they apply to my situation. How can I fix this error? Thanks.

sub isUndefOrWhitespace {
    my $str = '';
    $str = shift;
    
    print "$str\n";
    
    if (!defined $str) {
        return 0;
    }elsif (/^\s*$/) {
        return 0;
    }
    
    return 1;
}
[download]

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Re: Use of uninitialized value in pattern match (m//) by AnomalousMonk (Archbishop) on May 23, 2012 at 22:37 UTC
The regex `/^\s$/` matches by default against the `$_` scalar, which is nowhere initialized in the code you show. Perhaps `$str =~ /^\$/` was what you meant?	[reply] [d/l] [select]
Re: Use of uninitialized value in pattern match (m//) by tobyink (Canon) on May 23, 2012 at 22:38 UTC
It's the `elsif` condition that's triggering it. (The line numbers reported in Perl's error messages... well, there is a logic to them, but it can seem impenetrable at times. In multi-line constructs the line number reported is sometimes several lines away from where a human being would judge the error to have occurred.) When you use a pattern match (like `/^\s$/`) you need to tell it what variable to test. e.g.: `$str =~ /^\s$/` [download] If you do not tell it what variable to test, then Perl will test the special variable called `$_`. In your case, `$_` appears to be undefined. Hence the message. `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply] [d/l] [select]
Re: Use of uninitialized value in pattern match (m//) by sauoq (Abbot) on May 23, 2012 at 22:39 UTC
If this is the exact code you had on your windows box, you likely had a bug there too, even if you never noticed it. The problem looks like you are matching against `$_` in your eslif condition when you want to be matching against `$str`. Change that line to: `}elsif ($str =~/^\s*$/) {` [download] `-sauoq "My two cents aren't worth a dime.";`	[reply] [d/l] [select]
Re^2: Use of uninitialized value in pattern match (m//) by Sushma (Initiate) on Nov 09, 2012 at 09:47 UTC
hi, I am also getting same error for below code if ($response->{'Name'} =~ /Approval/) Could you pleas help me. Thanks, sushma	[reply]
Re^3: Use of uninitialized value in pattern match (m//) by Corion (Patriarch) on Nov 09, 2012 at 09:53 UTC
If you get that error, looking at perldiag usually tells you where the error comes from. In your case, `$response->{'Name'}` is not what you think it is. I recommend looking at `$response`, maybe by using Data::Dumper.	[reply] [d/l] [select]