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(bison.info)Grammar in Bison

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From Formal Rules to Bison Input
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   A formal grammar is a mathematical construct.  To define the language
for Bison, you must write a file expressing the grammar in Bison syntax:
a "Bison grammar" file.  Note: Bison Grammar Files.

   A nonterminal symbol in the formal grammar is represented in Bison
input as an identifier, like an identifier in C.  By convention, it
should be in lower case, such as `expr', `stmt' or `declaration'.

   The Bison representation for a terminal symbol is also called a
"token type".  Token types as well can be represented as C-like
identifiers.  By convention, these identifiers should be upper case to
distinguish them from nonterminals: for example, `INTEGER',
`IDENTIFIER', `IF' or `RETURN'.  A terminal symbol that stands for a
particular keyword in the language should be named after that keyword
converted to upper case.  The terminal symbol `error' is reserved for
error recovery.  Note: Symbols.

   A terminal symbol can also be represented as a character literal,
just like a C character constant.  You should do this whenever a token
is just a single character (parenthesis, plus-sign, etc.): use that
same character in a literal as the terminal symbol for that token.

   A third way to represent a terminal symbol is with a C string
constant containing several characters.  Note: Symbols, for more
information.

   The grammar rules also have an expression in Bison syntax.  For
example, here is the Bison rule for a C `return' statement.  The
semicolon in quotes is a literal character token, representing part of
the C syntax for the statement; the naked semicolon, and the colon, are
Bison punctuation used in every rule.

     stmt:   RETURN expr ';'
             ;

Note: Syntax of Grammar Rules.