Simple ANTLR4 grammar example

Hello.g4

// define a grammar called Hello
grammar Hello;
r   : 'hello' ID;
ID  : [a-z]+ ;
WS  : [ \t\r\n]+ -> skip ;

Hello.java

import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;

public class Hello {
  public static void main( String[] args) throws Exception 
  {
    HelloLexer lexer = new HelloLexer( new ANTLRFileStream(args[0]));
    CommonTokenStream tokens = new CommonTokenStream( lexer );
    HelloParser parser = new HelloParser( tokens );
    ParseTree tree = parser.r();
    ParseTreeWalker walker = new ParseTreeWalker();
    walker.walk( new HelloWalker(), tree );
  }
}

HelloWalker.java

public class HelloWalker extends HelloBaseListener {
  public void enterR(HelloParser.RContext ctx ) {
    System.out.println( "Entering R : " + ctx.ID().getText() );
  }

  public void exitR(HelloParser.RContext ctx ) {
    System.out.println( "Exiting R" );
  }
}

Makefile

all: Hello.class

HelloParser.java: Hello.g4
	antlr4 Hello.g4

Hello.class: HelloParser.java Hello.java HelloWalker.java
	javac Hello*.java

test.txt

hello world 

how to execute this program

@MarwanMedhat-97 check this out

Finally an actual simple example - much appreciated