CODEWITHSUNDEEP
antlr4
Paul Würtz
Paul Würtz

Reputation: 1771

ANTLR4: rule 'RULE' contains a closure with at least one alternative that can match an empty string

I am writing a file parser with ANTLR4. The file can have a number of blocks, which all begin and end with a (BEGIN | END) keyword. Here is a very simple example:

grammar test;

BEGIN: 'BEGIN';
END:   'END';
HEADER:'HEADER';
BODY:  'BODY';

file: block+;
ID: [A-Za-z];
NUM: [0-9];

block:
   | BEGIN HEAD statement* END HEAD
   | BEGIN BODY statement* END BODY
   ;
statement: ID '=' NUM;

The error thats get thrown is error(153): test.g4:8:0: rule file contains a closure with at least one alternative that can match an empty string , what I don't understand, since file has at least one empty block, with the begin-end style. Anyone sees what I am missing here?

Upvotes: 1

Views: 1448

Answers (1)

sepp2k
sepp2k

Reputation: 370445

block can match the empty string because there's nothing between the colon and the first |. Then in file, you use block+. This causes the error because you're applying + to something that can match the empty string, which could lead to an infinite looo that doesn't consume any input.

To fix this problem, just remove the first | in block.

Upvotes: 2

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