Validation Regex in UpdateFunctionConfiguration

[hodadako]

When updating function configuration using aws-cli, are there any specific reasons why hyphens(dashes, -) are not allowed in lambda environment variable keys?

botocore.exceptions.ClientError: An error occurred (ValidationException) when calling the UpdateFunctionConfiguration operation: 1 validation error detected: Value at 'environment.variables' failed to satisfy constraint: Map keys must satisfy constraint: [Member must satisfy regular expression pattern: [a-zA-Z]([a-zA-Z0-9_])+]