join lines after colon (perl)

Question

I have this lines:

alpha: beta
beta: alpha, beta
omega: beta, gamma, alpha
gamma: 
alpha
beta
gamma
epsilon: alpha

I want to join line contains

Answer 1

Here's a quick and dirty version:

#!/usr/bin/perl

use strict;
use warnings;

my $prevLine = "";
my @others;

for() {
    chomp;
    if (/:\s*$/) { # lines ends with : and potential space after
        $prevLine = $_;
    } elsif (!/:/) { # line doesn't contain ':'
        push(@others, $_);
    } elsif ($prevLine eq "") { # this is a "x: y, z" line, nothing in buffer
        print $_ . "\n";
    } else { # this is a "x: y, z" line, with a previous line in buffer
        print $prevLine . join(", ", @others) . "\n" . $_ . "\n";
    }
}

__DATA__
alpha: beta
beta: alpha, beta
omega: beta, gamma, alpha
gamma: 
alpha
beta
gamma
epsilon: alpha

Output:

alpha: beta
beta: alpha, beta
omega: beta, gamma, alpha
gamma: alpha, beta, gamma
epsilon: alpha