Extended BNF grammar optional [...] notation

Question

I read somewhere [x],y in this notation, x is optional, but I could not understand what is meant by optional. Does it means, there can be y or x,y or something else? Can anyone give some example about this?

Answer 1

Yes.

Documentation about []:

Optional: The items between [ and ] are optional. All or none of the items in the brackets are included.

In EBNF [] stands for optional parameters that can be ommitted, so [x],y defines the possibilities y and xy.

Example:

Bicycle_Accessories = saddle [bell | horn] {water_bottle_holders}

defines next possibilities:

saddle 
saddle bell 
saddle horn 
saddle water_bottle_holder 
saddle bell water_bottle_holder 
saddle bell water_bottle_holder water_bottle_holder