The Hime Parser Generator provides its own language for expressing context-free grammars. It is largely similar to the standard BNF form with some enhancements for the sake of expressivity.

// Single line comments begin with double slash
/*
 * This is a multiline comment
 */
grammar MathExp
{
    // The 'options' section in a grammar specifies various compilation options for the grammar.
    // At the very least this section may be empty.
    // For more information about this section, see: options reference
    options
    {
        // The Axiom option specifies the top rule for the grammar.
        Axiom = "exp";
        // The Separator option specifies the separator terminal (usually white space).
        Separator = "SEPARATOR";
    }
    // The 'terminals' section in a grammar specifies the lexical rules for terminals.
    // This section is optional.
    // For more information about this section, see: terminals reference
    terminals
    {
        // This is a lexical rule that defines the WHITE_SPACE terminal.
        // By convention, the name of terminals are generally UPPER_CASE.
        // U+XXXX represent the code of Unicode character.
        WHITE_SPACE -> U+0020 | U+0009 | U+000B | U+000C ;
        // This lexical rules reuses the previous definition of WHITE_SPACE.
        // Note that this rule defines the SEPARATOR terminal referred to in the Separator option above.
        // A terminal must be defined here before being used.
        SEPARATOR -> WHITE_SPACE+;
        // This set of three lexical rules defines the NUMBER terminal.
        // Their order of appearance is significant.
        INTEGER -> [1-9] [0-9]* | '0' ;
        // Now we can use INTEGER for the definition of REAL.
        REAL -> INTEGER? '.' INTEGER (('e' | 'E') ('+' | '-')? INTEGER)?
                    | INTEGER ('e' | 'E') ('+' | '-')? INTEGER ;
        // Now we can use both INTEGER and REAL for the definition of NUMBER.
        NUMBER -> INTEGER | REAL ;
    }
    
    // The 'rules' section in a grammar specifies the syntactic rules for variables.
    // At the very least this section may be empty.
    // For more information about this section, see: rules reference
    rules
    {
        // This is a syntactic rule that defines the exp_atom variable.
        // By convention, the name of variables are generally snake_case.
        // Note that the rule's definition refers to the NUMBER terminal.
        exp_atom -> NUMBER
                    | '(' exp ')' ;
        // The order of the syntactic rules is not significant.
        exp_factor -> exp_atom
                    | exp_factor '*' exp_atom
                    | exp_factor '/' exp_atom ;
        exp_term -> exp_factor
                    | exp_term '+' exp_factor
                    | exp_term '-' exp_factor ;
        exp -> exp_term ;
    }
}