Помогите написать регулярку. uptime и sed

$ uptime | sed 's/.*\(..:..:..\).*/\1/'
14:38:42

$ uptime | sed 's/.*up\s*\([^,]*\),.*/\1/'
4:32

uptime | sed 's/.*up\s*\([^,]*\),.*/\1/'

26 min

uptime -s

$ uptime --help

Usage:
 uptime [options]

Options:
 -h, --help     display this help and exit
 -s, --since    system up since
 -V, --version  output version information and exit

$ uptime -V
uptime from procps-ng 3.3.7

cdshines@v3700:~|⇒  uptime -V
uptime from procps-ng 3.3.3

-s, --since    system up since

cut -d' ' -f1 /proc/uptime
# кол-во секунд со старта системы

$ uptime
 00:44:33 up 5 days, 18:25,  5 users,  load average: 0,53, 0,55, 0,56

$ time1=$(sed -e "s/\..*//" /proc/uptime); convertsecs() {((h=${1}/3600)); ((m=(${1}%3600)/60)); ((s=${1}%60)); printf "%02d:%02d:%02d\n" $h $m $s;}; echo $(convertsecs $time1)
138:25:23

ivan@lefrat ~ $ uptime -s|sed 's/.* //'
18:40:41
ivan@lefrat ~ $ 

uptime -s | awk -F' ' '{ print $2 }'