#!/bin/bash
s='[Test][PR:Cook][Completed]
[Test][Finished][PR:Russell][Reviewer:SE]
[Dump][Reviewer:SE][Complete][PR:Arnold]'
grep -o '\[PR:[^]]*]' <<< "$s"
IyEvYmluL2Jhc2gKcz0nW1Rlc3RdW1BSOkNvb2tdW0NvbXBsZXRlZF0KW1Rlc3RdW0ZpbmlzaGVkXVtQUjpSdXNzZWxsXVtSZXZpZXdlcjpTRV0KW0R1bXBdW1Jldmlld2VyOlNFXVtDb21wbGV0ZV1bUFI6QXJub2xkXScKZ3JlcCAtbyAnXFtQUjpbXl1dKl0nIDw8PCAiJHMi