antlr4 - ANTLR 4.5 - Mismatched Input 'x' expecting 'x'

Question

I have been starting to use ANTLR and have noticed that it is pretty fickle with its lexer rules. An extremely frustrating example is the following:

grammar output;

test: FILEPATH NEWLINE TITLE ;

FILEPATH: ('A'..'Z'|'a'..'z'|'0'..'9'|':'|'\'|'/'|' '|'-'|'_'|'.')+ ;
NEWLINE: '
'? '
' ;
TITLE: ('A'..'Z'|'a'..'z'|' ')+ ;

This grammar will not match something like:

c:est.txt
x

Oddly if I change TITLE to be TITLE: 'x' ; it still fails this time giving an error message saying "mismatched input 'x' expecting 'x'" which is highly confusing. Even more oddly if I replace the usage of TITLE in test with FILEPATH the whole thing works (although FILEPATH will match more than I am looking to match so in general it isn't a valid solution for me).

I am highly confused as to why ANTLR is giving such extremely strange errors and then suddenly working for no apparent reason when shuffling things around.

Answer 1

This seems to be a common misunderstanding of ANTLR:

Language Processing in ANTLR:

The Language Processing is done in two strictly separated phases:

• Lexing, i.e. partitioning the text into tokens
• Parsing, i.e. building a parse tree from the tokens

Since lexing must preceed parsing there is a consequence: The lexer is independent of the parser, the parser cannot influence lexing.

Lexing

Lexing in ANTLR works as following:

• all rules with uppercase first character are lexer rules
• the lexer starts at the beginning and tries to find a rule that matches best to the current input
• a best match is a match that has maximum length, i.e. the token that results from appending the next input character to the maximum length match is not matched by any lexer rule
• tokens are generated from matches:
  • if one rule matches the maximum length match the corresponding token is pushed into the token stream
  • if multiple rules match the maximum length match the first defined token in the grammar is pushed to the token stream

Example: What is wrong with your grammar

Your grammar has two rules that are critical:

FILEPATH: ('A'..'Z'|'a'..'z'|'0'..'9'|':'|'\'|'/'|' '|'-'|'_'|'.')+ ;
TITLE: ('A'..'Z'|'a'..'z'|' ')+ ;

Each match, that is matched by TITLE will also be matched by FILEPATH. And FILEPATH is defined before TITLE: So each token that you expect to be a title would be a FILEPATH.

There are two hints for that:

• keep your lexer rules disjunct (no token should match a superset of another).
• if your tokens intentionally match the same strings, then put them into the right order (in your case this will be sufficient).
• if you need a parser driven lexer you have to change to another parser generator: PEG-Parsers or GLR-Parsers will do that (but of course this can produce other problems).