#!/bin/bash
s='[Test][PR:Cook][Completed]
[Test][Finished][PR:Russell][Reviewer:SE]
[Dump][Reviewer:SE][Complete][PR:Arnold]'
awk 'match($0,/\[PR:[^]]*]/) {print substr($0,RSTART,RLENGTH)}' <<< "$s"
IyEvYmluL2Jhc2gKcz0nW1Rlc3RdW1BSOkNvb2tdW0NvbXBsZXRlZF0KW1Rlc3RdW0ZpbmlzaGVkXVtQUjpSdXNzZWxsXVtSZXZpZXdlcjpTRV0KW0R1bXBdW1Jldmlld2VyOlNFXVtDb21wbGV0ZV1bUFI6QXJub2xkXScKYXdrICdtYXRjaCgkMCwvXFtQUjpbXl1dKl0vKSB7cHJpbnQgc3Vic3RyKCQwLFJTVEFSVCxSTEVOR1RIKX0nIDw8PCAiJHMi