The descriptions of lexical analysis and syntax use a modified BNF
grammar notation. This uses the following style of definition:
name: lc_letter (lc_letter | "_")*
lc_letter: "a"..."z"
The first line says that a name is an lc_letter followed by
a sequence of zero or more lc_letters and underscores. An
lc_letter in turn is any of the single characters `a' through `z'.
(This rule is actually adhered to for the names defined in lexical and
grammar rules in this document.)
Each rule begins with a name (which is the name defined by the rule)
and a colon. A vertical bar (|) is used to separate
alternatives; it is the least binding operator in this notation. A
star (*) means zero or more repetitions of the preceding item;
likewise, a plus (+) means one or more repetitions, and a
phrase enclosed in square brackets ([ ]) means zero or one
occurrences (in other words, the enclosed phrase is optional). The
* and + operators bind as tightly as possible;
parentheses are used for grouping. Literal strings are enclosed in
quotes. White space is only meaningful to separate tokens.
Rules are normally contained on a single line; rules with many
alternatives may be formatted alternatively with each line after the
first beginning with a vertical bar.
In lexical definitions (as the example above), two more conventions
are used: Two literal characters separated by three dots mean a choice
of any single character in the given (inclusive) range of ASCII
characters. A phrase between angular brackets (<...>) gives an
informal description of the symbol defined; e.g. this could be used
to describe the notion of `control character' if needed.
Even though the notation used is almost the same, there is a big
difference between the meaning of lexical and syntactic definitions:
a lexical definition operates on the individual characters of the
input source, while a syntax definition operates on the stream of
tokens generated by the lexical analysis. All uses of BNF in the next
chapter (``Lexical Analysis'') are lexical definitions; uses in
subsequent chapters are syntactic definitions.