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This document assumes some familiarity with context-free grammars. Please refer to your textbook or another resource, like Wikipedia's article about the subject.

How to write grammars

Grammars are written similarly to the usual presentation in textbooks. Each rule consists of a nonterminal symbol, an arrow, the symbols the rule produces, and then a period. Alternative productions for a nonterminal can be written using multiple statements or written together using a vertical bar.

A -> a B .
A -> a .
B -> A | b .

You can write rules over multiple lines -- spacing doesn't matter.

A -> b
     |
     A b .

Symbols are written like programming language identifiers. They can start with an ASCII letter, an underscore, or dollar sign. This can be followed by the same, plus numbers. For example:

A -> A_1 a .
A_1 -> $b _i18n .

Also, any quoted string can be a symbol. This is a valid grammar:

"音楽" -> "♫" "音楽" | .

Any symbol can be used as a nonterminal -- it just needs to be in the nonterminal position of a rule. (In other words, it's not required to capitalize a symbol for it to be a nonterminal.)

The first nonterminal is always the grammar's start symbol.

A rule that produces the empty string (sometimes written as A → ε) is just written with no symbols between the arrow and period. Grammophone will show this with the symbol ε.

A -> .

A symbol can't be an empty string. Grammophone won't accept this as a valid grammar:

# Invalid
A -> "" .

Instead of the arrow and period, you can use colon and semicolon, like in Bison. These are the same grammar:

A -> a | b .

A : a | b ;

You can add comments using the # character:

# This grammar is LL(1)

A -> b A | .

In Grammophone, the symbol Grammar.END is reserved for internal use.