Fixed position parsing - Lexing by character position in line.

It would be useful to be able to express character position constraints in a pure ANTLR grammar (without predicate actions).

Example:
I have lexical rules such as:
PS_BEGIN: {getCharPositionInLine()==23}? [bB];

Thanks for considering it. I understand it may be a specialized case, and should stay implemented as is. Just thought I'd suggest it

The issue names no files, tests, or implementation entry points. Start by reviewing how lexical rules and predicate actions are handled in ANTLR grammars; done would be a pure grammar way to express character-position constraints such as position 23 without predicate actions.

Written by the indexing model from the issue text.