What are the character restrictions for labels?

When working with Kubernetes labels, there are restrictions concerning the length and allowed values. Specifically, a valid label value:
1>must be 63 characters or less (can be empty),
2>unless empty, must begin and end with an alphanumeric character ([a-z0-9A-Z]),
3>can only contain dashes (-), underscores (_), dots (.), and alphanumerics.
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