Re^4: Regex match on implicit default variable ($

I am now able to exclude and reform $B, but I can't do it the other way (include only "def"). Isn't "while" suppose to be the opposite of "until"?

#!/usr/bin/perl

use strict;
use warnings;
use diagnostics;

my $B = "";
my $A = "abc
def
ghi
";

while ( $A =~ m{(.+)}g ) {
   $B .= "$1\n" unless $1 =~ /def/;
}
print "\$B = \n$B\n";


$B = "";
while ( $A =~ m{(.+)}g ) {
   $B .= "$1\n" while $1 =~ /def/;
}
print "\$B = \n$B\n";
[download]

$B =

abc

ghi

Use of uninitialized value $_ in pattern match (m//) at ./ExtractTest.pl line 21 (#1)

Now what am I doing wrong?

Comment on Re^4: Regex match on implicit default variable ($_) in a script, not a one liner Download Code

Replies are listed 'Best First'.

Re^5: Regex match on implicit default variable ($_) in a script, not a one liner
by Athanasius (Cardinal) on Oct 24, 2015 at 06:20 UTC

Isn't "while" suppose to be the opposite of "until"?

Yes, but you want the “opposite” (actually, the negation) of unless, which is simply if. See perlsyn#Statement-Modifiers.

But there is another problem: $1 refers to the first capture in the most recent regular expression match. In the expression:

$B .= "$1\n" unless $1 =~ /def/;
[download]

the concatenation $B .= "$1\n" occurs only if the regex match $1 ~ /def/ fails; in which case, there is no match, so the most recent match is still the one from the while loop regex. But in the expression:

$B .= "$1\n" if $match =~ /def/;
[download]

the concatenation occurs only if the new match succeeds; in which case, $1 is overwritten. You can fix this by assigning the value of $1 to a temporary variable:

while ($A =~ m{(.+)}g)
{
    my $capture = $1;
    $B .= "$capture\n" if $capture =~ /def/;
}
[download]

Hope that helps,

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

[reply]
[d/l]
[select]