Sometimes, however, we'd like to specify where in the string the regexp should try to match. To do this, we would use the anchor metacharacters ^ and $ . The anchor ^ means match at the beginning of the string and the anchor $ means match at the end of the string, or before a newline at the end of the string.

You also misplaced your parentheses assuming you do not wish to capture leading and trailing :. You will get your expected result with

$str = "R4:abcxyz45:LNX"; 
$str =~ /^[\w\d]+:([\w\d]+):\w+$/;
[download]

or

$str = "R4:abcxyz45:LNX"; 
$str =~ /[\w\d]+:([\w\d]+):\w+/;
[download]

or even

$str = "R4:abcxyz45:LNX"; 
$str =~ /:([\w\d]+):/;
[download]

... and simplifying even further to

$str =~ /:([\w]+):/;
[download]

Since \d is a subset of \w, [\w\d] is the same as [\w].

Then the square brackets are also not needed:

$str =~ /:(\w+):/;
[download]

my bad ... really my bad.silly mistake :). I got it. Thanks a lot Kenneth. Happy Weekend.

use YAPE::Regex::Explain;
my $re = '[\w\d]+(^:[\w\d]+:$)\w+';
print YAPE::Regex::Explain->new($re)->explain();

__END__

The regular expression:

(?-imsx:[\w\d]+(^:[\w\d]+:$)\w+)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  [\w\d]+                  any character of: word characters (a-z, A-
                           Z, 0-9, _), digits (0-9) (1 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    ^                        the beginning of the string
----------------------------------------------------------------------
    :                        ':'
----------------------------------------------------------------------
    [\w\d]+                  any character of: word characters (a-z,
                             A-Z, 0-9, _), digits (0-9) (1 or more
                             times (matching the most amount
                             possible))
----------------------------------------------------------------------
    :                        ':'
----------------------------------------------------------------------
    $                        before an optional \n, and the end of
                             the string
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  \w+                      word characters (a-z, A-Z, 0-9, _) (1 or
                           more times (matching the most amount
                           possible))
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

Also note that, since \d is a subset of \w, [\w\d] is the same as (and more simply coded as) [\w].

A regular expression isn't necessary. How about just using split with a ':' delimiter, then taking the second array element of the result?

main::(-e:1):   1
  DB<1> $str = "R4:abcxyz45:LNX"

  DB<2> @words = split(':',$str)

  DB<3> x $words[1]
0  'abcxyz45'
[download]

To just get one item, use list slice.

A very handy technique. Note that the order of the vars on the left don't have to be in the same order that they are in the input string (the indicies to the slice can be in any order).

I usually order the vars on the left to be in the order that they will be used in the code that follows the split. I fiddle with the indices on the right to make that possible. Also here I used a -1 instead of 2 just to show that is possible (-1 means the last thing in list)

Oh, PS, yes it is possible to write a single char like ':' to use in the split instead of /:/, but I always use the regex form.

#!/usr/bin/perl -w
use strict;

my $x = 'R4:abcxyz45:LNX';
my $server = (split(/:/,$x))[1];
print "$server\n";               #prints abcxyz45


(my $OS, $server) = (split(/:/,$x))[-1,1];
print "$OS   $server\n";      #prints LNX  abcxyz45
[download]

A regular expression is necessary because the first argument to split is a regular expression.

If you wanted to shorten up the regex, you could do something like this:

my $str = "R4:abcxyz45:LNX";
my ($servername) = ($str =~ /:(.+):/);
[download]

However, I should point out that this has a greater potential to create problems than the regexes offered by kennethk. In other words, the above has shorter keystrokes, but kennethk's solutions are safer.