ANTLR 4: Dynamic Token

Question

Given:

grammar Hbs;

var: START_DELIM ID END_DELIM;

START_DELIM: '{{';

END_DELIM: '}}';

I'd like to know how to change START_DELIM and END_DELIM at runtime to be for example <% and %>.

Does anyone know how to do this in ANTLR 4?

Thanks.

Answer 1

There's a way, but you'll need to tie your grammar to a target language (as of now, the only target is Java).

Here's a quick demo (I included some comments to clarify things):

grammar T;

@lexer::members {

  // Some default values
  private String start = "<<";
  private String end = ">>";  

  public TLexer(CharStream input, String start, String end) {
    this(input);
    this.start = start;
    this.end = end;
  }

  boolean tryToken(String text) {

    // See if `text` is ahead in the CharStream.
    for(int i = 0; i < text.length(); i++) {

      if(_input.LA(i + 1) != text.charAt(i)) {

        // Nope, we didn't find `text`.
        return false;
      }
    }

    // Since we found the text, increase the CharStream's index.
    _input.seek(_input.index() + text.length() - 1);

    return true;
  }
}

parse
 : START ID END
 ;

START
 : {tryToken(start)}? . 
   // The `.` is needed because a lexer rule must match at least 1 char.
 ;

END
 : {tryToken(end)}? .
 ;

ID
 : [a-zA-Z]+
 ;

SPACE
 : [ \t\r\n] -> skip
 ;

The { ... }? is a semantic predicate. See: https://github.com/antlr/antlr4/blob/master/doc/predicates.md

Here's a small test class:

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

public class Main {

  private static void test(TLexer lexer) throws Exception {
    TParser parser = new TParser(new CommonTokenStream(lexer));
    ParseTree tree = parser.parse();
    System.out.println(tree.toStringTree(parser));
  }

  public static void main(String[] args) throws Exception {

    // Test with the default START and END.
    test(new TLexer(new ANTLRInputStream("<< foo >>")));

    // Test with a custom START and END.
    test(new TLexer(new ANTLRInputStream("<? foo ?>"), "<?", "?>"));
  }
}

Run the demo as follows:

*nix

java -jar antlr-4.0-complete.jar T.g4
javac -cp .:antlr-4.0-complete.jar *.java
java -cp .:antlr-4.0-complete.jar Main

Windows

java -jar antlr-4.0-complete.jar T.g4
javac -cp .;antlr-4.0-complete.jar *.java
java -cp .;antlr-4.0-complete.jar Main

And you'll see the following being printed to the console:

(parse << foo >>)
(parse <? foo ?>)