A question on whitespace is difficult to address as one cannot see the objects of interest. Your regex suggests that in a multiline string you want to replace lines consisting only of whitespaces with truly empty lines. Whitespaces in non-empty lines will be preserved. I am replacing the whitespace with 'x' to see where we got matches: 

use strict;
use warnings;

my $string = "next line is spaces
      
next line is tabs
        
and now some newlines



end";

$string =~ s/^\s*$/x/mg;

print "$string\n";

[download]
The result of this would be: 
next line is spaces
x
next line is tabs
x
and now some newlines
xx
end

[download]
When you say you want to exclude newlines from the match, what is the desired effect you want to see? Preserve multiple empty lines?

In reply to Re: regex doubt on excluding by hdb
in thread regex doubt on excluding by Anonymous Monk

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