A Java regular expression matched against the entire name of each template VM on the configured node. The pattern controls how tight or loose the match is: jenkins-agent-.* matches every name with that prefix, while jenkins-agent-\d{4}-\d{2}-\d{2} matches only date-stamped names. The whole name must match, so use .* around a fragment to match it anywhere.

The expression is re-evaluated every time an agent is provisioned. When several templates match, the most recently created wins: highest ctime from the VM config's meta property; templates without a ctime (created before PVE 7.1, or restored from a backup) count as oldest; ties go to the highest VM id. This suits templates that are rebuilt on a schedule under fresh VM ids: new agents pick up the newest build automatically.

Provisioning fails when no template matches, so saving with zero current matches is allowed (the match count shown below this field is informational).