Rats! example

gistfile1.java

/**
* Syntax Analysis production
* void means do not create AST
* / means ordered alternative
*/

module Sample;

header { import java.util.*;}
body { public static final Set<String>
     KEYWORDS = new HashSet<String>(Arrays.asList("if", "else"));}

public void program = WS statement EOF; 

void statement = IF LPAREN expr RPAREN
                    statement ELSE statement
                 / IF LPAREN expr RPAREN statement
                 / SEMI;   
void expr = ID REL_OP ID; 

// simple production , are all void , and will be dropped in AST
void IF = "if"

void ELSE = "else"

void LPAREN = "("

void RPAREN = ")"

void SEMI = ";" 

// whitespace
void WS = ([ \t\n\f]/COMMENT)˚

/** &: positive predicate 
    !: negative predicate
    neither predicate will actually consume the character
**/
void COMMENT = "/*"(!"*"_/"*"!"/")˚"*/"

void EOF = !_;

void REL_OP = ("<="/"<"/">="/">") WS; // order alternation, cause "<" is the prefix of "<="

void ID = i:ID_INTERNAL WS
          &{!KEYWORDS.contains(i)};

void ID_INTERNAL = [a-zA-Z][a-zA-Z0-9]