10:04 Jan 3 

BTW, boldfacing used this way (to represent the literal characters) is a common convention in BNF and similar grammars.

09:59 Jan 3 

that's right !

09:57 Jan 3 

According to the text you quoted, the brackets in the line

pin_range ::= pin_name [ range_expression ] pin_name

should be bolded. Is that not the case?

09:46 Jan 3 

bolded text:
d) X X
e) RESULT
f) ,
g) BUS

09:23 Jan 3 

“It” means “the punctuation mark (vertical bar or bracket) itself”.

Since the vertical bar and brackets are special characters, when a rule needs to refer to the actual special character, the special will need to be escaped. In this text, the escape is performed by boldfacing (rather than by yet another special character).

09:18 Jan 3 

But what is bolded in the extract you have posted?

09:17 Jan 3 

"it" seems to mean "each of the item"

09:16 Jan 3 

A vertical bar separates alternative items (use one only) unless it appears in boldface, in which case
it stands for itself, e.g.,
timing_mode_operator ::= -> | <- | <-> | <-X-> | ->X<-
e) Square brackets enclose optional items unless it appears in boldface, in which case it stands for
itself. For example:
result_definition ::= RESULT ( [ result_sequence ] )
indicates result_sequence is an optional syntax item for result_definition, whereas
pin_range ::= pin_name [ range_expression ] pin_name
indicates square brackets are part of the syntax for pin_range.
f) Braces enclose a repeated item unless it appears in boldface, in which case it stands for itself. The
item may appear zero or more times; the repetitions occur from left to right as with an equivalent
left-recursive rule. Thus, the following two rules are equivalent:
comma_expression_list ::= expression { , expression }
comma_expression_list ::= expression | comma_expression_list , expression
g) Parenthesis enclose items within a group (use one only) unless it appears in boldface, in which case
it stands for itself. In the following example:
bus ::= ( BUS physical_name { attribute } ( { net_ref } | { pin_ref } { node_ref } ) )

09:12 Jan 3 

it would be interesting to have what follows "e.g.".
Grammatically speaking, "it" can only replace "a vertical bar".
In the second example, "it" does not even make sense in the sentence, as all nouns are plural.