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(python2.1-ref.info)Notation

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Notation
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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_letter's 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.