# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/^.*\shostname=(?:\S+\.)?([^\s.]+\.[^\s.]+)/*/gm;t' <<< "2021-07-20 18:19:14 reason=Not allowed to browse this category event_id=12345 protocol=HTTP action=Blocked transactionsize=16051 responsesize=789 requestsize=456 urlcategory=Blocked serverip=1.2.4.5 clienttranstime=0 requestmethod=GET refererURL=None useragent=Microsoft-Delivery location=Internal ClientIP=5.6.7.8 status=403 user=John url=dl.delivery.mp.microsoft.com/filestreamingservice/files/abcd-efgh-ijkl/pieceshash vendor=Zscaler hostname=dl.delivery.mp.microsoft.com
2021-07-20 18:19:14 reason=Not allowed to browse this category event_id=12345 protocol=HTTP action=Blocked transactionsize=16051 responsesize=789 requestsize=456 urlcategory=Blocked serverip=1.2.4.5 clienttranstime=0 requestmethod=GET refererURL=None useragent=Microsoft-Delivery location=Internal ClientIP=5.6.7.8 status=403 user=John url=dl.delivery.mp.microsoft.com/filestreamingservice/files/abcd-efgh-ijkl/pieceshash vendor=Zscaler hostname=google.com"
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html