ANTLR Is there a simple example

Question

I d like to get started with ANTLR  but after spending a few hours reviewing the examples at the antlr org site  I still can t get a clear understanding of the grammar to Java process   Is there some simple example  something like a four-operations calculator implemented with ANTLR going through the parser definition and all the way to the Java source code

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ANTLR mega tutorial by Gabriele Tomassetti is very helpful

It has grammar examples, examples of visitors in different languages (Java, JavaScript, C# and Python) and many other things. Highly recommended.

EDIT: other useful articles by Gabriele Tomassetti on ANTLR

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At https   github com BITPlan com bitplan antlr you ll find an ANTLR java library with some useful helper classes and a few complete examples  It s ready to be used with maven and if you like eclipse and maven   https   github com BITPlan com bitplan antlr blob master src main antlr4 com bitplan exp Exp g4  is a simple Expression language that can do multiply and add operations  https   github com BITPlan com bitplan antlr blob master src test java com bitplan antlr TestExpParser java has the corresponding unit tests for it   https   github com BITPlan com bitplan antlr blob master src main antlr4 com bitplan iri IRIParser g4 is an IRI parser that has been split into the three parts    parser grammar lexer grammar imported LexBasic grammar   https   github com BITPlan com bitplan antlr blob master src test java com bitplan antlr TestIRIParser java has the unit tests for it   Personally I found this the most tricky part to get right  See http   wiki bitplan com index php ANTLR maven plugin  https   github com BITPlan com bitplan antlr tree master src main antlr4 com bitplan expr  contains three more examples that have been created for a performance issue of ANTLR4 in an earlier version  In the meantime this issues has been fixed as the testcase https   github com BITPlan com bitplan antlr blob master src test java com bitplan antlr TestIssue994 java shows

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Note  this answer is for ANTLR3  If you re looking for an ANTLR4 example  then this Q amp A demonstrates how to create a simple expression parser  and evaluator using ANTLR4     You first create a grammar  Below is a small grammar that you can use to evaluate expressions that are built using the 4 basic math operators     -    and    You can also group expressions using parenthesis   Note that this grammar is just a very basic one  it does not handle unary operators  the minus in  -1 9  or decimals like  99  without a leading number   to name just two shortcomings  This is just an example you can work on yourself   Here s the contents of the grammar file Exp g   grammar Exp      This will be the entry point of our parser     eval          additionExp           Addition and subtraction have the lowest precedence     additionExp          multiplyExp                 multiplyExp              -  multiplyExp                        Multiplication and division have a higher precedence     multiplyExp          atomExp                atomExp                 atomExp                        An expression atom is the smallest part of an expression  a number  Or     when we encounter parenthesis  we re making a recursive call back to the    rule  additionExp   As you can see  an  atomExp  has the highest precedence     atomExp          Number              additionExp               A number  can be an integer value  or a decimal value    Number            0    9           0    9                We re going to ignore all white space characters    WS                    t      r     n     channel HIDDEN            Parser rules start with a lower case letter  and lexer rules start with a capital letter   After creating the grammar  you ll want to generate a parser and lexer from it  Download the ANTLR jar and store it in the same directory as your grammar file   Execute the following command on your shell command prompt   java -cp antlr-3 2 jar org antlr Tool Exp g   It should not produce any error message  and the files ExpLexer java  ExpParser java and Exp tokens should now be generated   To see if it all works properly  create this test class   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           parser eval              and compile it       nix MacOS javac -cp   antlr-3 2 jar ANTLRDemo java     Windows javac -cp   antlr-3 2 jar ANTLRDemo java   and then run it       nix MacOS java -cp   antlr-3 2 jar ANTLRDemo     Windows java -cp   antlr-3 2 jar ANTLRDemo   If all goes well  nothing is being printed to the console  This means the parser did not find any error  When you change  12  5-6   into  12  5-6  and then recompile and run it  there should be printed the following   line 0 -1 mismatched input   lt EOF gt   expecting       Okay  now we want to add a bit of Java code to the grammar so that the parser actually does something useful  Adding code can be done by placing   and   inside your grammar with some plain Java code inside it   But first  all parser rules in the grammar file should return a primitive double value  You can do that by adding returns  double value  after each rule   grammar Exp   eval returns  double value           additionExp        additionExp returns  double value           multiplyExp                 multiplyExp              -  multiplyExp                              which needs little explanation  every rule is expected to return a double value  Now to  interact  with the return value double value  which is NOT inside a plain Java code block        from inside a code block  you ll need to add a dollar sign in front of value   grammar Exp      This will be the entry point of our parser     eval returns  double value                                                             additionExp      plain code block     System out println  value equals     value                     Here s the grammar but now with the Java code added   grammar Exp   eval returns  double value           exp additionExp   value    exp value          additionExp returns  double value           m1 multiplyExp         value     m1 value                   m2 multiplyExp   value     m2 value                -  m2 multiplyExp   value -   m2 value                       multiplyExp returns  double value           a1 atomExp         value     a1 value                  a2 atomExp   value     a2 value                   a2 atomExp   value     a2 value                       atomExp returns  double value           n Number                  value   Double parseDouble  n text                 exp additionExp       value    exp value          Number            0    9           0    9             WS                    t      r     n     channel HIDDEN           and since our eval rule now returns a double  change your ANTLRDemo java into this   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        print the value           Again  re  generate a fresh lexer and parser from your grammar  1   compile all classes  2  and run ANTLRDemo  3        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   and you ll now see the outcome of the expression 12  5-6  printed to your console   Again  this is a very brief explanation  I encourage you to browse the ANTLR wiki and read some tutorials and or play a bit with what I just posted   Good luck   EDIT   This post shows how to extend the example above so that a Map lt String  Double gt  can be provided that holds variables in the provided expression   To get this code working with a current version of Antlr  June 2014  I needed to make a few changes  ANTLRStringStream needed to become ANTLRInputStream  the returned value needed to change from parser eval   to parser eval   value  and I needed to remove the WS clause at the end  because attribute values such as  channel are no longer allowed to appear in lexer actions

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version 4 7 1 was slightly different    for import   import org antlr v4 runtime      for the main segment - note the CharStreams   CharStream in   CharStreams fromString  12  5-6     ExpLexer lexer   new ExpLexer in   CommonTokenStream tokens   new CommonTokenStream lexer   ExpParser parser   new ExpParser tokens

User · Answer

For Antlr 4 the java code generation process is below -  java -cp antlr-4 5 3-complete jar org antlr v4 Tool Exp g   Update your jar name in classpath accordingly

[java] ANTLR: Is there a simple example?

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