Re: print identical keys once along with their values

[Anonymous Monk]

Why go to the trouble of using a hash?

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1146923

use strict;
use warnings;

$_ = join "\t", qw(1 A 1 B 2 G 2 H 2 V);

1 while s/\b((\d+)\s\S+)(.*?)\s\2\s(\S+)/$1:$4$3/;

print "$_\n";

[bsherkhane]

Thank you very much, can you explain me the regex in details , like what is \2 and $1 $4 $3 special variables purpose regards, Umesh

[Athanasius]

Hello bsherkhane,

The escape sequences \1, \2, \3, etc., are backreferences to captures in the current regex. The special variables $1, $2, $3, etc., are likewise backreferences to the captures in the most recent regex. $1 refers to the first capture, $2 to the second capture, and so on. Captures are numbered by counting left parentheses from the left. See perlre#Capture-groups.

The module YAPE::Regex::Explain is a useful tool for understanding regular expressions. Here is the explanation it gives for the left-hand side (i.e., the regex part) of the substitution in question:

Read more... (4 kB)

Hope that helps,