([^:]*):(.*)

sed 's/([^:]*):(.*)/1/' /etc/passwd

1st Capturing Group ([^:]*)

Match a single character not present in the list below [^:]*
* Quantifier — Matches between zero and unlimited times, as many times as possible, giving back as needed (greedy)
: matches the character : literally (case sensitive)
: matches the character : literally (case sensitive)
2nd Capturing Group (.*)
.* matches any character (except for line terminators)
* Quantifier — Matches between zero and unlimited times, as many times

sed 's/([^:]*):
          /([^:]*): Match the string till you see a colon `:` and group.

sed (.*)/1/' /etc/passwd
       (.*)  Match everything after `:` and group

sed 's/([^:]*):(.*)/12/' <<<"Hello:Unix:Users"
HelloUnix:Users

sed 's/([^:]*):(.*)/1;2/' <<<"Hello:Unix:Users"

sed 's/([^:]*):(.*)/1 Linux and 2/' <<<"Hello:Unix:Users"

(any symbol except : repeated zero or more times):(any symbol remeated zero or more times)

sed -r 's/([^:]*):(.*)/1/' /etc/passwd

root
daemon
bin
sys
sync
games