BusyBox v1.29.3 (2021-01-17 01:25:00 PST) multi-call binary on ESXi:
echo '{"expires": "Thu, 11 Oct 2012 11:30:29 +0000", "upload_id": "hhgJHflih753jDhhod", "offset": 293876}' | sed -n -e 's/.*"upload_id":\s*"*\([^"]*\)"*.*/\1/p' returns hhgJHflih753jDhhod.