#!/bin/bash
s='[Test][PR:Cook][Completed]
[Test][Finished][PR:Russell][Reviewer:SE]
[Dump][Reviewer:SE][Complete][PR:Arnold]'
sed -n 's/.*\(\[PR:[^]]*]\).*/\1/p' <<< "$s"
IyEvYmluL2Jhc2gKcz0nW1Rlc3RdW1BSOkNvb2tdW0NvbXBsZXRlZF0KW1Rlc3RdW0ZpbmlzaGVkXVtQUjpSdXNzZWxsXVtSZXZpZXdlcjpTRV0KW0R1bXBdW1Jldmlld2VyOlNFXVtDb21wbGV0ZV1bUFI6QXJub2xkXScKc2VkIC1uICdzLy4qXChcW1BSOlteXV0qXVwpLiovXDEvcCcgPDw8ICIkcyI=