5.1 Syntax Productions
In this International Standard, formal syntax is described in a manner similar to ISO 8879 with the following exceptions.
A sequence of expressions indicates that the expressions shall occur in the order shown.  The , operator is not used.
The occurrence indicators ?, +, and * have higher precedence than sequencing, which in turn has higher precedence than the connectors | and &. For example,
a b  | c d*
is equivalent to
(a b)  | (c (d*))
A syntactic-literal is indicated by a monospaced typeface as shown.
In a syntax production, double square brackets ( ) can be used to surround an or group.  The meaning of this is similar to an and group.  However, if any of the members of the or group have a * or + occurrence indicator, then they can occur the number of times indicated but intermixed with other members of the group.  For example,
  a* | b+ | c | d?
means a sequence containing only a's, b's, c's, and d's in which any number of a's occur, one or more b's, exactly one c, and at most one d.