What restrictions exist for the name field of a channel namespaces?

Channel namespace can contain any unicode character (including e.g. spaces), with the following exceptions:

• They cannot start with a square bracket [
  Square brackets are reserved as a means to provide metadata or instructions to Ably about the intended channel when subscribing.
• They cannot contain a colon :
  Colons are used to delimit channel namespaces. Find out more about channel namespaces
• They cannot be empty.