Question

I’m using \D to not display digits but why the digits are being displayed using perl regular expressions?

Here’s the content of the text2.tx file

1. Hello Brue this is a test.
2. Hello Lisa this is a test.
This is a test 1.
This is a test 2.

Here is the perl program.

#!/usr/bin/perl

use strict;
use warnings;

open READFILE,"<", "test2.txt" or die "Unable to open file";

while(<READFILE>)
{
   if(/\D/)
   {
      print;
   }
}

Answer 1

Lets take a look at what is going on behind the scenes. Your while loop is equivalent to:

while(defined($_ = <READFILE>))
{
    if($_ =~ /\D/)
    {
        print $_;
    }
}

So, you are checking if the line contains a non-digit character (which it does) and then printing that line.

If you want to print Hello Brue this is a test. instead of 1. Hello Brue this is a test., then you would have to use something like:

while(<READFILE>) {
    s/^\d+\. //;
    print;
}

Also, it would make for more readable code if you used a variable rather than $_.