$ cat string
#! /usr/bin/ksh

string="[1] private.enterprises.954.1.1.1.1.1.2618 \(OctetString\): U [2] private.enterprises.954.1.1.1.1.2.2618 \(OctetString\): 2618"
echo string = $string
string=${string#*: }
first=${string%% *}
second=${string##*: }
echo first = $first
echo second = $second
exit 0
$ ./string
string = [1] private.enterprises.954.1.1.1.1.1.2618 \(OctetString\): U [2] private.enterprises.954.1.1.1.1.2.2618 \(OctetString\): 2618
first = U
second = 2618