Are there regulations about wearing a headscarf?

Yes is the short answer and the circumstances seems to vary. For some airlines e.g. Lion Air in Indonesia it is part of the uniform. Other airlines e.g. Emirates accommodate individuals, so if your headscarf is worn as part of your religion, they will give you an Emirates hijab, which is white with the Emirates logo on it. Some Airlines ask to sign an agreement form stating you would be willing to remove any religious headwear in the event of an emergency. It’s really best to check beforehand with the airline you are planning to apply to, they may have details on their website or you can email them your query.