How do I find character positions in ANTLR 2?

Question

I have a simple grammar, and have produced a pair of c# classes using antlr 2.7.7. When the parser finds an error with a token, it throws an exception; I want to find out how many characters into a parsed stream the token came. How do I do that?

Answer 1

It's been a long time ago since I played with ANTLR, but if I remember well, to do what you want, I had to subclass the parser to keep a counter of characters that was incremented each time a new token was found (with the token length of course).

Answer 2

You ought to read chapter 10 ("Error Reporting and Recovery") from Terrence Parr's book "The Definitive ANTLR Reference".

Not knowing what target language you're using, it'll be hard to tell you exactly what to do. But I'll assume you're using the Java target, and you can correct me if I'm wrong.

When an ANTLR recognizer fails to match an input string, it throws a very specific exception, based on the failure context. (There are nine different kinds of exceptions, RecognitionException is the root type, and it has eight subclasses of its own: MismatchedTokenException, MismatchedTreeNodeException, NoViableAltException, EarlyExitException, FailedPredicateException, MismatchedRangeException, MismatchedSetException, MismatchedNotSetException).

The root exception type (RecognitionException) has a few handy public fields that you might want to take a look at (specifically: "index", "line" and "charPositionInLine"). The "index" field tells you the exact character position where the error was found. The "line" and "charPositionInLine" fields are pretty self-explanatory. Here's the JavaDoc:

http://www.antlr.org/api/Java/classorg_1_1antlr_1_1runtime_1_1_recognition_exception.html