in reply to Regex matching a string beginning with "root: " but not containing another string

You can do it with negative lookahead: 
/^root: (?!\Qmyself@domain.of-my.own\E)/;

[download]
You could also do it with two regexps: 
/^root: /g and !/\G\Qmyself@domain.of-my.own\E/g;

[download]
Caution: Contents may have been coded under pressure.

In Section Seekers of Perl Wisdom