How Can I Use ANTLR to Parse and Evaluate Simple Mathematical Expressions?

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ANTLR: A Simple Step-by-Step Guide with Example

ANTLR, or ANother Tool for Language Recognition, is a powerful parser generator tool that allows you to define grammars and generate parsers in various programming languages. If you're a beginner, you may find the ANTlr.org examples a bit overwhelming. This guide provides a simplified example to help you understand how ANTLR works.

Creating the Grammar File

To start, define a simple grammar in a text file, e.g., "Exp.g". The following grammar lets you evaluate basic mathematical expressions with four operators ( , -, *, /) and parentheses:

grammar Exp;

eval
    : additionExp EOF
    ;

additionExp
    : multiplyExp 
         ( '+' multiplyExp 
         | '-' multiplyExp
         )* 
    ;

multiplyExp
    : atomExp
         ( '*' atomExp 
         | '/' atomExp
         )* 
    ;

atomExp
    : Number
    | '(' additionExp ')'
    ;

Number
    : ('0'..'9')+ ('.' ('0'..'9')+)?
    ;

Generating the Lexer and Parser

Assuming you have the ANTLR jar downloaded, run the following command to generate the lexer (ExpLexer.java) and parser (ExpParser.java) classes:

java -cp antlr-3.2.jar org.antlr.Tool Exp.g

Creating the Test Class

Now, create a test class, e.g., ANTLRDemo.java, that invokes the parser and processes the expression:

import org.antlr.runtime.*;

public class ANTLRDemo {
    public static void main(String[] args) throws Exception {
        ANTLRStringStream in = new ANTLRStringStream("12*(5-6)");
        ExpLexer lexer = new ExpLexer(in);
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        ExpParser parser = new ExpParser(tokens);
        System.out.println(parser.eval());
    }
}

Enhancing the Grammar with Java Code

To perform calculations, add Java code blocks within your grammar rules, enclosed in { }. Here's the modified grammar:

grammar Exp;

eval
    : exp=additionExp {$value = $exp.value;}
    ;

additionExp
    : m1=multiplyExp       {$value =  $m1.value;} 
         ( '+' m2=multiplyExp {$value += $m2.value;} 
         | '-' m2=multiplyExp {$value -= $m2.value;}
         )* 
    ;

// ...

Running the Example

Compile all generated and test classes, then run the ANTLRDemo class:

// *nix/MacOS
java -cp antlr-3.2.jar org.antlr.Tool Exp.g   // 1
javac -cp .:antlr-3.2.jar ANTLRDemo.java      // 2
java -cp .:antlr-3.2.jar ANTLRDemo            // 3

// Windows
java -cp antlr-3.2.jar org.antlr.Tool Exp.g   // 1
javac -cp .;antlr-3.2.jar ANTLRDemo.java      // 2
java -cp .;antlr-3.2.jar ANTLRDemo            // 3

This will calculate and display the result of "12*(5-6)" to your console.

This simple example demonstrates the basics of ANTLR. To learn more, explore the ANTLR wiki, tutorials, and documentation.

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