Parsing unstructured text with ANTLR - parsing

Question

As an example, lets say I want to parse mostly unstructured text with single markup element, double star **. This is my ANTLR grammar:
text : (plain | tag)+ ;
plain : ~(TAG) ;
tag : TAG tag_inner TAG ;
tag_inner : ~(TAG) ;
TAG : '**' ;
TEXT : ('a'..'z' | ' ' | '.')+ ;
This grammar works just fine if the text I'm parsing is syntactically correct, that is for every opening ** there is a closing **. If there is an odd number of **s, ANTLR complains, and errors out.
How would one fix this, so that ANTLR will look ahead for a closing double star, and if there is no one treat that lone double star as plain text? I'm pretty sure ANTLR can do this and that syntactic/semantic predicates are the answer, but after an our spent reading the docs, I still can't work it out.

Answer 1

This will get messy when you expand your grammar! :)
But, sure, it is possible using predicates. Here's a demo:
T.g
grammar T;
options {
output=AST;
}
tokens {
ROOT;
PROPER_TAG;
}
parse
: text+ EOF -> ^(ROOT text+)
;
text
: (tag)=> tag // syntactic predicate here! (the `(...)=>`)
| plain
| TAG
;
plain
: ~TAG
;
tag
: TAG plain TAG -> ^(PROPER_TAG plain)
;
TAG : '**' ;
TEXT : ('a'..'z' | ' ' | '.')+ ;
Main.java
import org.antlr.runtime.*;
import org.antlr.runtime.tree.*;
import org.antlr.stringtemplate.*;
public class Main {
public static void main(String[] args) throws Exception {
TLexer lexer = new TLexer(new ANTLRStringStream("this **is** just **a simple** demo **."));
TParser parser = new TParser(new CommonTokenStream(lexer));
CommonTree tree = (CommonTree)parser.parse().getTree();
DOTTreeGenerator gen = new DOTTreeGenerator();
StringTemplate st = gen.toDOT(tree);
System.out.println(st);
}
}
Run the demo
java -cp antlr-3.3.jar org.antlr.Tool T.g
javac -cp antlr-3.3.jar *.java
java -cp .:antlr-3.3.jar Main
which will produce some DOT-output that corresponds to the following AST:
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