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

This also works if the input is provided in a file as an argument to the script, and is a little shorter:

$/=$.;$_=<>;s/\s?\n/, /g;s/, (\w+:),?/\n$1/g;s/, $/\n/;print

It's not very readable, but it works with strict and warnings.

Output:

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

It can probably be shortened even further. -ape or #!perl -ap would be a good start.