if ($tainted =~ / ^              # don't start mid-string
                  \s*            # skip any leading space
                  (\w{3}[\w.]*)  # prefix to $1
                  \@
                  ([\w.]+)       # hostname to $2
                  \s*            # skip any trailing space
                  $ /x)          # don't end mid-string
    { $untainted = "$1\@$2"; }