基本语法：四个语言模式

four abstract computer language patterns.

• Sequence: This is a sequence of elements such as the values in an array initializer.
• Choice: This is a choice between multiple, alternative phrases such as the different kinds of statements in a programming language.
• Token dependence: The presence of one token requires the presence of its counterpart elsewhere in a phrase such as matching left and right parentheses.
• Nested phrase: This is a self-similar language construct such as nested arithmetic expressions or nested statement blocks in a programming language.

To implement these patterns, we really only need grammar rules comprised of alternatives, token references, and rule references(Backus-Naur-Format [BNF]).

1. Sequence

subrule operator	meaning	sample
+	one or more elements	(INT)+ INT+
*	Zero or more	INT*
?	optional constructs ('='expr)? is the same as ('='expr|)

• sequence with terminator and sequence with separator

file : (row '\n')* ;       // sequence with a '\n' terminator
row : field (',' field)* ; // sequence with a ',' separator
field: INT ; // assume fields are just integers 

2. Choice

   Sample:

field : INT | STRING ;
stmt: node_stmt | edge_stmt | attr_stmt | id '=' id | subgraph ;

3. Token Dependency

   use a sequence that specifies both symbols, usually enclosing or grouping other elements.

   Sample: vector : '[' INT+ ']' ; // [1], [1 2], [1 2 3], ...

4. Nested Phrase

   stat: 'while' '(' expr ')' stat     // match WHILE statement
    | '{' stat* '}'  // match block of statements in curlies
   

   ​

高级语法：嵌入代码

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