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          


        

Answer 1

This was not directly OP's problem, but for those who have the same error message, here is something you could check.

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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.

Answer 2

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).