$ cat passwd
root:x:0:0:ifeuu1 root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
noncom:x:0:0:cause RegEx Fail:/noncom:/bin/bash
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown

##</code><code>##

#!/usr/bin/perl
# perl-grep4.pl

my $pattern = shift @ARGV;

my $regex = eval { qr/$pattern/ };
die "Check your pattern! $@" if $@;

while( <> )
    {
    if( m/$regex/ )
        {

        print "$_";

        print "\t\t\$&: ",
            substr( $_, $-[0], $+[0] - $-[0] ),
            "\n";

        foreach my $i ( 1 .. $#- )
            {
            print "\t\t\$$i: ",
                substr( $_, $-[$i], $+[$i] - $-[$i] ),
                "\n";
            }
        }
    }

##</code><code>##

$ Test5.pl '(?<!\broot:)x:0:' passwd
noncom:x:0:0:cause RegEx Fail:/noncom:/bin/bash
                $&: x:0:

##</code><code>##

PW:root:passwd:(?<!\broot:)x:0:

##</code><code>##

#!/usr/bin/perl
#use strict;
#use warnings;
use Text::ParseWords;
my ($RECORD, @FIELDS, $RE);
my $DFile="/home/infosec/data/RegEx";
open (DATA, "<$DFile") || die ("Cannot open DATA file \n");
my @LINES=<DATA>;
my $tLINES=@LINES;
foreach $RECORD (@LINES[0..$tLINES-1]) {
  @FIELDS=split(/:/, "$RECORD");
  my $TNAME=$FIELDS[0];
  my $TVALU=$FIELDS[1];
  my $TFILE=$FIELDS[2];
  my $REGEX=$FIELDS[3];
  chomp($REGEX);
  my $regex = eval { qr/$REGEX/ };
  print "$regex \n";
  open (TFILE, "<$TFILE") || die ("Cannot open Test file \n");
  while (<TFILE>)
    {
    if( m/$regex/ )
        {

        print "$_";

        print "\t\t\$&: ",
            substr( $_, $-[0], $+[0] - $-[0] ),
            "\n";

        foreach my $i ( 1 .. $#- )
            {
            print "\t\t\$$i: ",
                substr( $_, $-[$i], $+[$i] - $-[$i] ),
                "\n";
            }
        }
    }
  print "\n";
}
print "\n";

##</code><code>##

$ Test6.pl

root:x:0:0:ifeuu1 root:/root:/bin/bash
                $&:
bin:x:1:1:bin:/bin:/sbin/nologin
                $&:
noncom:x:0:0:cause RegEx Fail:/noncom:/bin/bash
                $&:
daemon:x:2:2:daemon:/sbin:/sbin/nologin
                $&:
adm:x:3:4:adm:/var/adm:/sbin/nologin
                $&:
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
                $&:
sync:x:5:0:sync:/sbin:/bin/sync
                $&:
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
                $&: