sed expression to extract group of the first match

With GNU grep, assuming there's at most one START...END per line:

grep -oPm1 'STARTK.*?(?=END)' file

With awk, assuming the first occurrence is on the first line and that there's no END before the first START and that there's no START in the value to be returned:

awk -F'START|END' '{print $2;exit}' file

or

awk -F'START|END' '$0 ~ FS {print $2;exit}' file

With perl:

perl -lne 'do{ print $1; last } if /START(.*?)END/' file

Output

firstgroup

sed -nr '/.*START(.*)END.*/{s/.*START(.*)END.*/1/p;q}'
# or (probably faster)
sed -nr 's/.*START(.*)END.*/1/p; t quit; b end; : quit; q; : end;'