grammar

grammar is a shunting-yard based expression parser to generate an Abstract Syntaxt Tree (AST). At its heart it is a library to allow you to quickly create your own expression language. But off course, since it is so easy, grammar also ships with some default expression parsers and evaluators for basic math and boolean support.

The tokenizer, parser and also evaluators are highly customizable. For example, you can configure the operators, the terminal symbols and customize about almost everything else.

Out of the box math and boolean parser/evaluator support

grammar is able to evaluate expressions like:

Boolean expression examples

res = eval("!(true | false) | (false & true)")

res = eval("not(true or false) or (false and true)")

Math expression examples

res = eval("10 + 5.6 / 20 * (40 % 2)")

Variable support

If your expression contains variables, grammar supports them aswell:

EvalContext<Double> context = new EvalContext<>();
context.setVariable("alpha", 15d);
res = eval("90 - alpha", context);

Just include a EvalContext<Double> context in your eval argument, and variable-reference nodes are substituted for you.

Custom Parser

This is an example of the default math expression parser configuration - you can create one for your own needs:

Math Parser

public class MathExpressionParser extends ExpressionParser {

    public MathExpressionParser(){
        super(new OperatorSet(
                new Operator("+", 2, true, Arity.Binary),
                new Operator("-", 2, true, Arity.Binary),
                new Operator("*", 3, true, Arity.Binary),
                new Operator("/", 3, true, Arity.Binary),
                new Operator("^", 4, false, Arity.Binary),
                new Operator("%", 4, false, Arity.Binary),

                new Operator("+", 99, true, Arity.Unary),
                new Operator("-", 99, true, Arity.Unary)
                        ),
                new Token(TokenType.Whitespace, " "),
                new Token(TokenType.Whitespace, "\t"),
                new Token(TokenType.Parentheses_Open, "("),
                new Token(TokenType.Parentheses_Closed, ")"),
                new Token(TokenType.Whitespace, ",")

        );
    }
}

Boolean Parser

public class BooleanExpressionParser extends ExpressionParser  {

    public BooleanExpressionParser(){
        super(new OperatorSet(
                    new Operator("&", 3, true, Arity.Binary, "and"),
                    new Operator("|", 3, true, Arity.Binary, "or"),
                    new Operator("^", 3, true, Arity.Binary, "xor"),
                    new Operator("!", 99, true, Arity.Unary, "not", "~")
                ),
                new Token(TokenType.Literal, "true"),
                new Token(TokenType.Literal, "false"),
                new Token(TokenType.Whitespace, " "),
                new Token(TokenType.Whitespace, "\t"),
                new Token(TokenType.Parentheses_Open, "("),
                new Token(TokenType.Parentheses_Closed, ")")
        );
    }
}

How to get started

You should have a look at our Unit tests here to get a feeling how to use this library.