<abc>                                     xy*
xyz: x  |  y  | z                         xy+
abc: a || b  || c                         xy?
def: [d | e] f                            xy {1,4}

<abc>

A value of type abc

xyz: x | y | z

Indicates alternatives

abc: a || b || c

Indicates some combination in any order

def: [d | e] f

Groups items so means d or e followed by f

xy*

Zero or more times

xy+

One or more times

xy?

Optional

xy {1,4}

At least 1 and not more than 4