ANTLR 4.5 - Mismatched Input 'x' expecting 'x'
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).
This was not directly OP's problem, but for those who have the same error message, here is something you could check.
I had the same Mismatched Input 'x' expecting 'x' vague error message when I introduced a new keyword. The reason for me was that I had placed the new key word after my VARNAME lexer rule, which assigned it as a variable name instead of as the new keyword. I fixed it by putting the keywords before the VARNAME rule.