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    my $re = shift or die "re expected\n";
    while (<>) {
...
        my ($field, $value) = ($_ =~ $re);
        print "'$_' -> ('$field', '$value')\n";
    }

 'a:b:c' -> ('a', 'b:c')
 'a\:b:c' -> ('a\:b', 'c')
...
 'a\\\:b:c' -> ('a\\\:b', 'c')
 'a:' -> ('a', '')
 ':b' -> ('', 'b')

 
 'a\:b' -> ('', 'b')

 ^((?:[^:\\]+|\\.)*):(.*)